Add subspan() to SkSpan

Change-Id: I1ac2ec8cf22a96c50261ca3863edeb0ed3c24dd6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/287579
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed