Add GL_APPLE_sync support

While debugging Flutter iOS issues, I realized that they weren't getting
cross context images (we were just passing around codec backed images).

Bug: skia:
Change-Id: I6190b24eb281e839659d28767f9a0b31c05f13e0
Reviewed-on: https://skia-review.googlesource.com/20450
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed