This will be required for async readback support.
This is implemented using sync objects when available and otherwise
calls glFinish.
Relaxes the unit test requirement that providing a callback with no
work to flush always calls the proc immediately.
Bug: skia:8962
Change-Id: Ieefcab6dccc3924e50260343f01904e7303bb12b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212198
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
b0d8b76c4f