[graphite] Suppress alpha-only image shader logic via runtime effects

This makes results consistent with Ganesh and CPU, where
alpha-only images produce {0,0,0,a} when sampled in raw
mode, or from runtime effects.

Change-Id: I48d1912c4a745cffd8081b3299eff129f56cb3bf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/759516
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
2 files changed