html <canvas> is used to process multiple tiledImages to blend
is a specific way.
(special handling,
when compositeOperation is 'source-over' and opacity is 1,
useSketch is false, otherwise useSketch is true
)
Valid values are 'source-atop', 'source-in', 'source-out',
'destination-over', 'destination-atop', 'destination-in',
'destination-out', 'lighter', 'copy' or 'xor'
http://www.w3schools.com/tags/canvas_globalcompositeoperation.asp