[graphite] Add Swizzle::selectChannelInR(i) helper

Bug: b/333909822
Change-Id: I1694b59fe43e9c4289ef2c1fdb882f4524710900
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/840277
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
1 file changed