The split composite shader is now quite similar to a normal
brush shader (uses prim header, supports normal clip masks
etc). The only real difference is that it draws a quad polygon
from local points instead of a rectangle. It should be easy
to unify this into the brush_image shader in the future.

Add a temporary "ClipNodeCollector" interface to correctly
handle clips that are ancestors of the current rasterization
root. Although it's not enforced by the API, I have verified
that any time a clip from a spatial node that is an ancestor
of the current rasterization root is applied on all primitives
that are part of that picture. This means we can safely apply
those clips during compositing of the rasterization root rather
than on each individual primitive. This fixes correctness
issues with clips across rasterization roots, and is also a
quite significant performance win in some cases, since the
clip mask is now applied once when the rasterization root
is drawn, rather than on each indidvidual primitive.

The end result of this is actually quite good - it will allow
us to simplify some of the clipping code in the future, and is
a performance win. The implementation leaves a lot to be
desired, but gets the required result without having to fix
up both gecko and wr clipping API mismatches. The long term
solution for this is to update gecko and WR to make better use
of clip chains on stacking contexts, rather than putting
clips from the stacking context on each individual primitive (we
have discussed doing this previously, but it's low-ish priority
for now given other bugs).