[graphite] Support .000r read-swizzle (to be used for alpha-only)

Change-Id: I4f0b810927db79d93f5077ea9136128150332b85
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/759557
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed