[canvaskit] add a gm for testing all blendmodes

Bug: skia:10264
Change-Id: I0b903798e5ebbd379df69268f2c78f381262c856
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/291176
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed