[graphite] Add TestingImageProvider

This ImageProvider-derived class provides basic conversion and caching functionality for out internal tools

Bug: b/238756324
Change-Id: I2dd2bc5d3e9cbb3458541da17e76c8d4eafa83d0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/565160
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed