Add a common way to make rect op for testing that uses a GP with a local
matrix.
Change-Id: I958d1230bd5067b2e4b60fcd374e2f7718681e43
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/255782
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
