Release YUVA planes in SkImage_GpuYUVA after flattenning to RGBA.

Also removed unused virtual function from SkImage_GpuBase and override
on SkImage_GpuYUVA.

Change-Id: Ib47b46b529b16976181cb9453976133d66e0f952
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212734
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
5 files changed