Adds the basic infrastructure for caching the outputs of a
rasterized Picture in the texture cache.

For now, this only applies if the following conditions are met:
 * The picture has a blur filter on it (includes text-shadows).
 * The contents of the picture and clips are in the root
   coordinate system (no rotations / perspective etc).

This covers most real-world use cases of blurs and text-shadows.

Future work:
 * Expand to cache other Picture types (e.g. drop-shadows, other filters).
 * Support caching of complex coordinate system pictures which
   are not animating (and thus worth caching).
 * Support deep comparison of picture elements between display lists.
   (caching only applies during scrolling in this first pass).