Picture shader resource caching.

Replace the current/naive shader caching mechanism with a more general
implementation based on SkResourceCache.

Caching the bitmap shader itself (as opposed to just the tile bitmap)
makes for a chunkier key, but OTOH avoids allocating new shaders on
cache hit.

R=reed@google.com,mtklein@google.com

Review URL: https://codereview.chromium.org/671683004
2 files changed