Make SkMask immutable, introduce SkMaskBuilder

Change-Id: Iac8b937e516dbfbbcefef54360dd5b7300bacb67
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/733844
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
49 files changed