Add correct bias to triangle drawing

[lkesteloot]

The fpgasim code computes a bias for each edge, in order to guarantee that pixels aren't ever drawn twice. We don't do that in the FPGA code. See whether we need to, then add it. (It's pretty simple I think.)

[bradgrantham]

This probably is only important when drawing with blending, which we don't support. Otherwise pixels from a second triangle sharing an edge with a first triangle will just replace the first triangle's pixels or be discarded, depending on their Z value. That's my take on it.