gl_graphics_pipeline: Improve shader builder synchronization using fences (#7969)

* 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