Convert GPU path handling to using prepareForDrawing

Small cleanup in the mask case to remove cast.

Change-Id: I0867fa80372cc917eae7c1ca84beafd699d8a349
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207705
Auto-Submit: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed