llvmpipe: calculate more accurate interpolation value at origin

Some rounding errors could crop up when calculating a0. Use a more accurate
method (barycentric interpolation essentially) to fix this, though to fix
the REAL problem (which is that our interpolation will give very bad results
with small triangles far away from the origin when they have steep gradients)
this does absolutely nothing (actually makes it worse). (To fix the real
problem, either would need to use a vertex corner (or some other point inside
the tri) as starting point value instead of fb origin and pass that down to
interpolation, or mimic what hw does, use barycentric interpolation (using
the coordinates extracted from the rasterizer edge functions) - maybe another
time.)
Some (silly) tests though really want a high accuracy at fb origin and don't
care much about anything else (Just. Don't. Ask.).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

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