add gm to test backdrop blurs and clipping/hintrects

Bug: skia:8783
Change-Id: I27907673d4a5e1f8c5c0b26c704da367e0437a79
Reviewed-on: https://skia-review.googlesource.com/c/194190
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
2 files changed