Use GrRenderTargetContext::readPixels in GrTextureToYUVPlanes

This is salvaged from an ancient CL or mine. Thoughts?

Change-Id: I3712ca911d1ab5def3f4dcb080a3f9e226cdd44b
Reviewed-on: https://skia-review.googlesource.com/7031
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed