Treat all alpha-only images equally

We were inconsistent about special treatment for kAlpha_8 vs. other
alpha-only color types (other code paths were already adjusted).
Hopefully, we can remove this special behavior entirely, but as long
as we have this code, let's be somewhat principled.

Change-Id: I373a3c5e8c917d911b55b265631ca964c780a31d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/472996
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed