Make sure to unbind xfer buffer before glReadPixels to cpu call.

If follow on change when I try to use cached buffers for read backs we
get into a state where we're doing normal readPixels but we still have
the buffer alive and it is still bound to PIXEL_PACK_BUFFER which causes
INVALID_OPERATION errors.

Bug: skia:11226
Change-Id: If5486cc2743908ee7222925afb372c1cfafef429
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/366319
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed