We want to give SkPixelRef a way to signal over to GrResourceCache that it's become...

We want to give SkPixelRef a way to signal over to GrResourceCache that it's become pointless to keep around textures based on that SkPixelRef when its pixels change, so that it can be a good citizen and free those textures.

This adds an invalidation listener mechanism to SkPixelRef to let it send this message while still staying ignorant of who's listening.

These messages are tricky to deliver.  The SkPixelRefs they originates from and the GrResourceCaches they ultimately end up at may be on different threads; neither class is threadsafe; their object lifetimes are totally independent; it's a many-senders-to-many-receivers relation; and neither codebase should really know about the other.

So I've added a per-message-type global message bus to broadcast messages to threadsafe inboxes.  Anyone can post() a message, which will show up in all the inboxes of that type, read whenever the inbox's owner calls poll().  The implementation is _dumb_; it can be improved in several dimensions (inbox size limits, lock-free message delivery) if we find the need.

I took some care to make sure not to send the invalidation message for any SkPixelRef that's sharing a generation ID with another SkPixelRef.

BUG=
R=bsalomon@google.com, scroggo@google.com, reed@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/26734003

git-svn-id: http://skia.googlecode.com/svn/trunk@11949 2bbb7eff-a529-9590-31e7-b0007b416f81