asahi: skeleton for tess

This implements a rough skeleton of what's needed for tessellation. It contains
the relevant lowerings to merge the VS and TCS, running them as a compute
kernel, and to lower the TES to a new VS (possibly merged in with a subsequent
GS). This is sufficient for both standalone tessellation and tess + geom/xfb
together. It does not yet contain a GPU accellerated tessellator, simply falling
back to the CPU for that for now. Nevertheless the data structures are
engineered with that end goal in mind, in particular to be able to tessellate
all patches in parallel without needing any prefix sums etc (using simple
watermark allocation for the heap).

Work on fleshing out the skeleton continues in parallel. For now, this does pass
the tests and lets the harder stuff get regression tested more easily. And
merging early will ease rebase.