Convert the CPU mask case to use prepareForDrawing

Change-Id: I3a36084544e12730f4815dbf5b6c78a1cd719f1b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207761
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed