LibSoftGPU: Use float instead of int for triangle screen coords

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.

Userland/Libraries/LibSoftGPU/Config.h

@@ -17,7 +17,6 @@ namespace SoftGPU {
 static constexpr bool ENABLE_STATISTICS_OVERLAY = false;
 static constexpr int NUM_SAMPLERS = 2;
 static constexpr int MILLISECONDS_PER_STATISTICS_PERIOD = 500;
-static constexpr int SUBPIXEL_BITS = 5;
 static constexpr int NUM_LIGHTS = 8;
 
 // See: https://www.khronos.org/opengl/wiki/Common_Mistakes#Texture_edge_color_problem