Instead, we provide access to both the clip node and scroll
node for a given primitive in the shader. The shader then
calculated the appropriate local clip rect.

There's a few hacks in here - for example, the way that
I pack the clip/scroll IDs into a single u32 for the
shaders. In the future, we'll be switching the vertex formats
to work on u16 size data fields, so this will become a lot
simpler then.