* gl_graphics_pipeline: Improve shader builder synchronization
Make use of GLsync objects to ensure better synchronization between shader builder threads and the main context
* gl_graphics_pipeline: Make built_fence access threadsafe
* gl_graphics_pipeline: Use GLsync objects only when building in parallel
* gl_graphics_pipeline: Replace GetSync calls with non-blocking waits
The spec states that a ClientWait on a Fence object ensures the changes propagate to the calling context
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