Add dm via-option `rtblend-` to test using Runtime Blends.

Change-Id: I46844754a86ae44b61747b7244edf6cc1fd73d61
Bug: skia:12080
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/429102
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed