llvmpipe: do final the pixel in/out triangle test in the fragment shader

llvmpipe: do final the pixel in/out triangle test in the fragment shader

The test to determine which of the pixels in a 2x2 quad is now done in
the fragment shader rather than in the calling C code.  This is a little
faster but there's a few more things to do.

Note that the step[] array elements are in a different order now.  Rather
than being in row-major order for the 4x4 grid, they're in "quad-major"
order.  The setup of the step arrays is a little more complicated now.
So is the course/intermediate tile test code, but some lookup tables
help with that.

Next steps:
 - early-cull 2x2 quads which are totally outside the triangle.
 - skip the in/out test for fully contained quads
 - make the in/out comparison code tighter/faster.