llvmpipe: Calculate fixed point coordinates for triangle setup earlier.

llvmpipe: Calculate fixed point coordinates for triangle setup earlier.

This allows us to calculate the triangle's area using fixed point,
previously it was cacluated in floating point space. It was possible
that a triangle which had negative area in floating point space had
a positive area in fixed point space.

Fixes fdo 40920.

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