Convert the fallback source case to use prepareForDrawing

Change-Id: I15438ecf7acba9e6b47c8de8b1acb8b24da5b8e9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207708
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed