[graphite] Add initial version of SkPictureShader

Please see skbug.com/13875 (Remove SkPictureShader). If we were able to remove SkPictureShader we could simplify SkKeyContext but, right now, we're stuck.

Bug: b/255993547

Change-Id: I549036133b7422f7f86ad2184d68a07f89a08efd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/596619
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
10 files changed