Add support for brushes to be drawn as a list of segments that
are part of a larger primitive. Initially, these segments are
nine-patches only, although they may be expanded in the future
to work with generic segment rects.

This allows large brush primitives to be split into a number
of segments that can be split among the opaque and alpha passes,
which means that large rectangles are often rendered with the
inner rectangle as part of the opaque pass. Additionally, edge
segments are rendered in the opaque pass when the primitive does
not have a rotation or perspective transform applied.

When a rectangle is split, the clip tasks are generated only
where required for individual segments. This can significantly
reduce the amount of allocated area for clip masks, and also
the amount of GPU time spent drawing clip masks.

Previously, rectangle splitting could only work on LocalClip
items specified as part of the item, which meant that rectangles
in Gecko would never be split. Now, we calculate splitting
during prepare_prim_for_render, consulting the current clip chain
for the primitive. This means that we get the benefits of rectangle
splitting in Gecko, which doesn't use LocalClip API.

Having rectangle splitting handled as segments also means that we
don't add extra top-level primitives that take extra time to do
visibility culling on, saving some CPU time.

Future work:
 - Support more cases of primitive splitting.
 - Port images and gradients to be brushes so they automatically get splitting.