[graphite] Add test for static vertices padding.

Change-Id: Ia4307e28e6eb57c62384e3579afa580e38a464af
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/995237
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Thomas Smith <thomsmit@google.com>
6 files changed