Add reverse iterators to SkSpan

Change-Id: Ie4002a97fca860f0096ad9114f707ba8476aabb4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/267936
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed