Convert GPU SDF handling to use prepareForDrawing

Change-Id: Ic8af9d608e5c31ed59e37f28e74e0f39e80f0fdf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207707
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed