This replaces the fixed point subpixel precision logic.
GLQuake now effectively renders artifact-free. Previously white/gray
pixels would sometimes be visible at triangle edges, caused by slightly
misaligned triangle edges as a result of converting the vertex window
coordinates to `int`. These artifacts were reduced by the introduction
of subpixel precision, but not completely eliminated.
Some interesting changes in this commit:
* Applying the top-left rule for our counter-clockwise vertices is now
done with simpler conditions: every vertex that has a Y coordinate
lower than or equal to the previous vertex' Y coordinate is counted
as a top or left edge. A float epsilon is used to emulate a switch
between `> 0` and `>= 0` comparisons.
* Fog depth calculation into a `f32x4` is now done once per triangle
instead of once per fragment, and only if fog is enabled.
* The `one_over_area` value was previously calculated as `1.0f / area`,
where `area` was an `int`. This resulted in a lower quality
reciprocal value whereas we can now retain floating point precision.
The effect of this can be seen in Tux Racer, where the ice reflection
is noticeably smoother.
We now have one set of texture coordinates per texture unit.
Texture coordinate generation and texture coordinate assignment is
currently only stubbed. This will be rectified in another commit.
