Add a real implementation for gm_knowledge.h This depends on
the presence of files in the form $GMK_DIR/foo/{max,min}.png
The implementation also writes out failures in a report directory.
Add a utility: experimental/make_gmkb which is a stand-alone
go executable that generates the foo/{max,min}.png data.
tools/skqp/README.md has instructions on running SkQP.
Also: add SkFontMgrPriv.h
Change-Id: Ibe1e9a7e7de143d14eee3877f5f2d2d8713f7f49
Reviewed-on: https://skia-review.googlesource.com/65380
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
