Adding observer pattern to SkPixelRef in order to propagate notifyPixelsChanged

This change is require in order to implement the code review comment posted here: http://codereview.appspot.com/5430058/

The comment in question was:
"I think this change is too broad, for all raster devices to be marked volatile.
We call notifyPixelsChanged() on each draw to the device, so perhaps you can use
the generationID or some other signal."

Using notifyPixelsChanged is actually a much more efficient approach because it allows the the flushing of deferred draw commands to be postponed until the source image data is actually about to be invalidated. A little extra plumbing is necessary to make that happen, hence this change.

[junov1, 2011-12-05 21:49:28]

PTAL

[reed1, 2011-12-05 22:04:17]

Can the previous CL be updated to reflect the current state of the code, + this
proposed CL? That would help me to see the context where this requested change
fits in.

[junov1, 2011-12-06 16:44:49]

Closing this review. changes will be integrated into this CL moving forward:

http://codereview.appspot.com/5430058/