Add unit test confirming Runtime Blends match native blends.

We perform the same draw operation twice--once with a `setBlendMode`
based blend, and then once more with `setBlender` and passing SkSL which
is equivalent to the native blend op. If this test fails, it would
indicate that Runtime Blends aren't being applied appropriately (or
perhaps that a GPU's built-in blending modes are cutting corners?).

Change-Id: I4ca1f9ed600d9ec733687cc6de7d3e2eb6e765c9
Bug: skia:12080
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/428318
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
2 files changed