This patch hooks up the primitive interning work with the tile
caching work.

Since we have a uid that guarantees uniqueness of the content of
primitives and clip nodes, we can use that to construct a
(reasonably) efficient hash key for the content of a tile.

Along with some additional information (such as the state of the
transforms used by the tile), we can construct a hash key for
the content of a tile that is stable across new display lists,
even if the shape of the clip-scroll tree changes in ways that
don't affect the tile.

This patch takes advantage of that to retain tiles when a new
scene is built, where the content of a tile would result in
the same output.