The previous version of the function didn't generate correct wide lines
of even width.
The most notable changes are:
* Make variable names far more descriptive about the process.
* Corrected the width calculation; we should deduct one pixel from the
width because the pixels at the center of the line doesn't count for
the triangles.
* Now we calculate *two* ratios, one for left/top displacement (dxmin)
and one for right/bottom (dxmax), this fix the behavior with lines
of even width.
It can probably be optimized.
The (previously refactored) polygon_generic function didn't draw
consistent polygons (equilateral polygons were not equilateral nor
symmetrical).
The most notable changes are:
* The horizontal edges are searched for when finding the polygon
boundaries, drawn and discarded from the edge list used to detect
intersections.
* The intersections are now checked and calculated from the current value
of the scan line (ymin) instead of in the middle (ymin + 0.5).
* Because the change in the scan line behavior, we should duplicate
the intersections in the maximum Y value of an edge or there will be
draw errors with concave and complex polygons.
* The rounds of the X coordinates in the hline function calls are
switched to draw the inner pixels.
* Removed the ugly micro-optimization of qsort at the end.
This implementation of the scan line algorithm may not be technically
correct, it's not optimized and it have problems with some edge cases,
like a wide line from (x0, y) to (x1, y + 1), therefore it should be
reviewed in the future.
The rounds used in the library are towards positive/negative infinity.
Since we're in a cartesian plane, rounds around zero are much more
convenient, so we can use positive and negative values with consistent
results.
The functions `polygon8`, `polygon32` and `polygon32rgba` all have exactly
the same logic in code, only changes the hline function called inside.
Now all the logic is contained in `polygon_generic` which gets a
function pointer to the hline handler, so there is no duplicated code
anymore.
