Remove alpha readback path in GrGLGpu::onReadPixels

Change-Id: I13d5a885a5a016ca7555702c5798c5b3f31b1cc8
Reviewed-on: https://skia-review.googlesource.com/131820
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed