Add debug-trace ops to SkRasterPipeline.

These ops follow closely in the footsteps of SkVM, and the test cases
are adapted directly from SkVMTest.cpp.

Change-Id: I3dc826d897ddc9d4861d5c68c0744c1296361488
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/660379
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed