Suppress alpha-only image shader logic when sampled via runtime effects

Change-Id: Ib832a7ee127aedb56f99a0cacd1bf5a146eb5cd8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/756397
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
12 files changed