Respect ImageSetEntry alpha in one-at-a-time code path

Previously it was ignored. Update GM with test case that covers this.

Change-Id: I93e7de285a9d4020dd48e14e09e55fdfb346fff7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/242083
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed