Move this code back out to C for now, will generate separately.
Shader now takes a mask parameter instead of C0/C1/C2/etc.
Shader does not currently use that parameter and rasterizes whole
pixel stamps always.
Currently counting number of tris, how many tiles of each size are
fully covered, partially covered or empty, etc.
Set LP_DEBUG=counters to enable. Results are printed upon context
destruction.