notify the surface when the canvas draws into it

Add caching of the snapshot image form a surface
Notify the surface when the canvas draws into it, so it can invalidate the
cached image, and (if needed) perform a copy-on-write on the surface if it
was being shared with the image.
Committed: https://code.google.com/p/skia/source/detail?r=5306

[reed1, 2012-08-08 17:35:11]

70% ready to commit...

[reed1, 2012-08-27 21:02:48]

ready to commit, so others can help finish (e.g. gpu backend). Should not get
exercised by any clients (yet).

[epoger, 2013-01-02 18:54:52]

https://codereview.appspot.com/6441115/diff/6001/gyp/core.gyp
File gyp/core.gyp (right):

https://codereview.appspot.com/6441115/diff/6001/gyp/core.gyp#newcode20
gyp/core.gyp:20: '../src/image',
So our "core" package now depends on code within the "image" package?  This
breaks our current method of importing our code into google3/...

[epoger, 2013-01-02 19:53:47]

On 2013/01/02 18:54:52, epoger wrote:
> https://codereview.appspot.com/6441115/diff/6001/gyp/core.gyp
> File gyp/core.gyp (right):
> 
> https://codereview.appspot.com/6441115/diff/6001/gyp/core.gyp#newcode20
> gyp/core.gyp:20: '../src/image',
> So our "core" package now depends on code within the "image" package?  This
> breaks our current method of importing our code into google3/...

It turns out that it's not as dire as my comment indicates.

This doesn't really break our method of importing Skia into google3/third_party.
 What it *does* do is effectively make "core" depend on "image", even though
that dependency is not expressed in core.gyp.  So our google3/third_party BUILD
file now needs to build "image", when it didn't before.

From discussion with Mike, it sounds like that will be OK.