ANGLE previously used the RGBA passthrough shader for ALPHA copies. This
commit adds a dedicated ALPHA shader that will not pass RGB components
when doing an ALPHA copy.

BUG=angleproject:2239
TEST=angle_end2end_tests
Change-Id: I52d1f18a8f968db005cd887803123c17e16df938
Reviewed-on: https://chromium-review.googlesource.com/758717
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>