Combine mask loops in bitmap device

Change-Id: I9d32394f14353a8a0c2c46c270a072f8fd9024e4
Reviewed-on: https://skia-review.googlesource.com/c/165182
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed