SkQP: make_gmkb, gm_knowledge (GM Knowledgebase)

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>
20 files changed