Reland: 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: I50f15ecd029b28b69c0f68dc4126df3a4dd61d75
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268685
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
9 files changed