Allow clients to specify an external SkImageFilter cache.

Allow clients to specify an external SkImageFilter cache.

This change allows external callers to substitute their own
SkImageFilter cache for the default intra-frame cache in Skia. This
allows the caller to perform inter-frame caching for example, by the
maintaining a persistent cache between frames and doing custom
invalidation.

R=reed@google.com

Committed: https://code.google.com/p/skia/source/detail?r=14181

[Stephen White, 2014-04-11 19:10:21]

reed@: PTAL. Thanks!

[reed1, 2014-04-11 19:56:34]

Why is this associated with the canvas, and not (e.g.) global, or picture or
surface or thread-local? What are the keys into the cache?

[Stephen White, 2014-04-11 20:05:54]

On 2014/04/11 19:56:34, reed1 wrote:
> Why is this associated with the canvas, and not (e.g.) global, or picture or
> surface or thread-local? What are the keys into the cache?

It could be any of those, really. A global would be fine. The key is the same as
the intra-frame cache (the SkImageFilter), with the wrinkle that the caller has
to set the relevant cache before drawing a particular primitive. Blink already
maintains such a cache per-primitive, so it's easy to hook in.

Note that this won't be used in impl-side painting, so threading is not an
issue. It's only used to preserve existing partial invalidation caching
functionality in Blink until those platforms can be switched over to impl-side
painting. At that point, they will lose frame-to-frame caching, but gain
parallelism and GPU rasterization, which for most use cases leaves them ahead
(or we figure out how to do partial-invalidation caching in Skia).

[Stephen White, 2014-04-14 15:16:30]

New patch: moved the external cache to a class-static (global) on SkImageFilter.

[reed1, 2014-04-14 15:20:28]

lgtm

[Stephen White, 2014-04-14 15:51:51]

Committed patchset #2 manually as r14181 (presubmit successful).