Don't change mask format in generateImage.

Change-Id: I879e5ace9bff4ea6edc5e522d3c48a4edd0ea242
Reviewed-on: https://skia-review.googlesource.com/c/174064
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
2 files changed