Split codec-related parts out of tools/Resources.h

This will make tests easier to modularize since only a few
unit tests actually need to decode an image to work.

Change-Id: I1ded07f22b4bcbf356a36f7b3b4fd56d21c828cf
Bug: b/40045064
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/763560
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
109 files changed