Initial checkin of GM verifier framework

The goal of the verifier framework is to enable opt-in checks of the
images produced by individual GMs. The basis of verification will be
comparing the rendered output of a GM against a source-of-truth image,
such as one generated by the CPU backend.

In the short term this can enable coarse-grained sanity checks for a
subset of GMs to catch e.g. egregious rendering bugs. In the longer term
this can provide an SkQP-style suite of tests that can be run across
many/all GMs to provide a vote of confidence in the rendering
correctness of new devices.

Bug: skia:9855
Change-Id: Id7310de8005ffa7e8eb2fd0e4008f5f8db1419ab
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/267761
Commit-Queue: Derek Sollenberger <djsollen@google.com>
Auto-Submit: Tyler Denniston <tdenniston@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
8 files changed