Added option zoomToRefPoint for all gestureSettings

This is an option for different applications or user preferences:
If zoomToRefPoint in a gesture setting is true (default), the zoom is
centered at the pointer position. Otherwise, the zoom is centered at the
canvas center.

src/openseadragon.js

@@ -314,6 +314,8 @@
   * @property {Boolean} [gestureSettingsMouse.dblClickToZoom=false] - Zoom on double-click gesture. Note: If set to true
   *     then clickToZoom should be set to false to prevent multiple zooms.
   * @property {Boolean} [gestureSettingsMouse.pinchToZoom=false] - Zoom on pinch gesture
+  * @property {Boolean} [gestureSettingsMouse.zoomToRefPoint=true] - If zoomToRefPoint is true, the zoom is centered at the pointer position. Otherwise, 
+  *     the zoom is centered at the canvas center.
   * @property {Boolean} [gestureSettingsMouse.flickEnabled=false] - Enable flick gesture
   * @property {Number} [gestureSettingsMouse.flickMinSpeed=120] - If flickEnabled is true, the minimum speed to initiate a flick gesture (pixels-per-second)
   * @property {Number} [gestureSettingsMouse.flickMomentum=0.25] - If flickEnabled is true, the momentum factor for the flick gesture
@@ -326,6 +328,8 @@
   * @property {Boolean} [gestureSettingsTouch.dblClickToZoom=true] - Zoom on double-click gesture. Note: If set to true
   *     then clickToZoom should be set to false to prevent multiple zooms.
   * @property {Boolean} [gestureSettingsTouch.pinchToZoom=true] - Zoom on pinch gesture
+  * @property {Boolean} [gestureSettingsTouch.zoomToRefPoint=true] - If zoomToRefPoint is true, the zoom is centered at the pointer position. Otherwise, 
+  *     the zoom is centered at the canvas center.
   * @property {Boolean} [gestureSettingsTouch.flickEnabled=true] - Enable flick gesture
   * @property {Number} [gestureSettingsTouch.flickMinSpeed=120] - If flickEnabled is true, the minimum speed to initiate a flick gesture (pixels-per-second)
   * @property {Number} [gestureSettingsTouch.flickMomentum=0.25] - If flickEnabled is true, the momentum factor for the flick gesture
@@ -338,6 +342,8 @@
   * @property {Boolean} [gestureSettingsPen.dblClickToZoom=false] - Zoom on double-click gesture. Note: If set to true
   *     then clickToZoom should be set to false to prevent multiple zooms.
   * @property {Boolean} [gestureSettingsPen.pinchToZoom=false] - Zoom on pinch gesture
+  * @property {Boolean} [gestureSettingsPan.zoomToRefPoint=true] - If zoomToRefPoint is true, the zoom is centered at the pointer position. Otherwise, 
+  *     the zoom is centered at the canvas center.
   * @property {Boolean} [gestureSettingsPen.flickEnabled=false] - Enable flick gesture
   * @property {Number} [gestureSettingsPen.flickMinSpeed=120] - If flickEnabled is true, the minimum speed to initiate a flick gesture (pixels-per-second)
   * @property {Number} [gestureSettingsPen.flickMomentum=0.25] - If flickEnabled is true, the momentum factor for the flick gesture
@@ -350,6 +356,8 @@
   * @property {Boolean} [gestureSettingsUnknown.dblClickToZoom=true] - Zoom on double-click gesture. Note: If set to true
   *     then clickToZoom should be set to false to prevent multiple zooms.
   * @property {Boolean} [gestureSettingsUnknown.pinchToZoom=true] - Zoom on pinch gesture
+  * @property {Boolean} [gestureSettingsUnknown.zoomToRefPoint=true] - If zoomToRefPoint is true, the zoom is centered at the pointer position. Otherwise, 
+  *     the zoom is centered at the canvas center.
   * @property {Boolean} [gestureSettingsUnknown.flickEnabled=true] - Enable flick gesture
   * @property {Number} [gestureSettingsUnknown.flickMinSpeed=120] - If flickEnabled is true, the minimum speed to initiate a flick gesture (pixels-per-second)
   * @property {Number} [gestureSettingsUnknown.flickMomentum=0.25] - If flickEnabled is true, the momentum factor for the flick gesture
@@ -1044,6 +1052,7 @@ function OpenSeadragon( options ){
                 clickToZoom: true,
                 dblClickToZoom: false,
                 pinchToZoom: false,
+                zoomToRefPoint: true,
                 flickEnabled: false,
                 flickMinSpeed: 120,
                 flickMomentum: 0.25,
@@ -1054,6 +1063,7 @@ function OpenSeadragon( options ){
                 clickToZoom: false,
                 dblClickToZoom: true,
                 pinchToZoom: true,
+                zoomToRefPoint: true,
                 flickEnabled: true,
                 flickMinSpeed: 120,
                 flickMomentum: 0.25,
@@ -1064,6 +1074,7 @@ function OpenSeadragon( options ){
                 clickToZoom: true,
                 dblClickToZoom: false,
                 pinchToZoom: false,
+                zoomToRefPoint: true,
                 flickEnabled: false,
                 flickMinSpeed: 120,
                 flickMomentum: 0.25,
@@ -1074,6 +1085,7 @@ function OpenSeadragon( options ){
                 clickToZoom: false,
                 dblClickToZoom: true,
                 pinchToZoom: true,
+                zoomToRefPoint: true,
                 flickEnabled: true,
                 flickMinSpeed: 120,
                 flickMomentum: 0.25,

src/viewer.js

@@ -2652,7 +2652,7 @@ function onCanvasClick( event ) {
         if ( gestureSettings.clickToZoom ) {
             this.viewport.zoomBy(
                 event.shift ? 1.0 / this.zoomPerClick : this.zoomPerClick,
-                this.viewport.pointFromPixel( event.position, true )
+                gestureSettings.zoomToRefPoint ? this.viewport.pointFromPixel( event.position, true ) : null
             );
             this.viewport.applyConstraints();
         }
@@ -2691,7 +2691,7 @@ function onCanvasDblClick( event ) {
         if ( gestureSettings.dblClickToZoom ) {
             this.viewport.zoomBy(
                 event.shift ? 1.0 / this.zoomPerClick : this.zoomPerClick,
-                this.viewport.pointFromPixel( event.position, true )
+                gestureSettings.zoomToRefPoint ? this.viewport.pointFromPixel( event.position, true ) : null
             );
             this.viewport.applyConstraints();
         }
@@ -3005,7 +3005,9 @@ function onCanvasPinch( event ) {
                 panByPt.y = 0;
             }
             this.viewport.zoomBy( event.distance / event.lastDistance, centerPt, true );
+            if ( gestureSettings.zoomToRefPoint ) {
                 this.viewport.panBy(panByPt, true);
+            }
             this.viewport.applyConstraints();
         }
         if ( gestureSettings.pinchRotate ) {
@@ -3068,7 +3070,7 @@ function onCanvasScroll( event ) {
                 factor = Math.pow( this.zoomPerScroll, event.scroll );
                 this.viewport.zoomBy(
                     factor,
-                    this.viewport.pointFromPixel( event.position, true )
+                    gestureSettings.zoomToRefPoint ? this.viewport.pointFromPixel( event.position, true ) : null
                 );
                 this.viewport.applyConstraints();
             }