What is this monstrosity? Final Fantasy VIII relies on X coordinates
being truncated during scanline drawing, with negative coordinates
becoming positive and vice versa. Fortunately the bits that we need
are consistent across the entire polygon, so we can get away with
truncating the vertices. However, we can't do this to all vertices,
because other game's vertices break in various ways. For example,
+1024 becomes -1024, which is a valid vertex position as the ending
coordinate is exclusive. Therefore, 1024 is never truncated, only
1023. Luckily, FF8's vertices get culled as they do not intersect
with the clip rectangle, so we can do this fixup only when culled,
and everything seems happy.
Reduces the code size by roughly half, which may be beneficial on
icache-starved processors.
Also fixes too-bright values being fed into the blending equation.
