This patch adds the infrastructure required to allow render tasks
to be rendered into the texture cache, where they can be stored
and referenced across multiple frames.

Since the cache key is based on the contents of the task itself,
the cache items remain valid even when a new display list is provided,
so long as the content of the cached task is the same. When the cache
is determined to be valid, the render task is just drawn as a simple
image, rather than executing the complex render task chain used to
build the output.

Convert box-shadows to make use of this new feature. When a new
box-shadow is required, the render task chain is set up to draw the
minimal box-shadow, and blur it. The blurred result is then stored in
the texture cache, available for use on this and subsequent frames.

Since items are drawn into the texture cache, this will also help
with batching in the future, as we collapse more items to be able to
use a simple shader that draws an image from the texture cache.

Remove blur regions since they are no longer used or required.
Remove old render task sharing within a single frame, it's redundant now.

TODO (as future work):
 * Support caching of RGBA8 render tasks.
 * Convert other tasks (e.g. clips, drop-shadows) to use cacheable tasks.
 * Use this as basis for optimizing rendering of static Pictures (e.g. a picture with a filter chain + image).