External SkImageGenerator API

Introduce an SkImageGenerator API to support the implementation of
externally-managed image decode and scale caches.

BUG=skia:5806
R=reed@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4720

Change-Id: Ibfe37af5471f78f28f88f9d5e80938882be1a344
Reviewed-on: https://skia-review.googlesource.com/4720
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
27 files changed
tree: a082a45e7672eeeb94c5a58084091cc9f07bf685
  1. recipe_modules/
  2. recipes.py
  3. recipes/