[Impeller] use glFenceSync to correctly synchronize background image decoding. #158963
Description
Taking a quick look at Skia, it looks like they handle this by using glFenceSync semaphores and glFlush
If the platform does not support glFenceSync (like GLES 2.0), then it looks like they need to fall back to glFinish
(https://skia.googlesource.com/skia/+/refs/heads/main/src/gpu/ganesh/gl/GrGLGpu.cpp#4307)