Bresenham triangle filling

[spearson78]

Rather than trying to fix the existing algorithm I have migrated to a bresenham based triangle fill. This resolves all the weird rendering issues included the wide filled triangles at the 12:00 and 06:00 and makes the filled triangles identical to the non filled version.

I unfortunately had to make major changes to the line drawing to make it possible to draw 3 lines concurrently. I extracted the state of the line drawing to a struct to enable this. The triangle filling also requires that all lines are drawn in y++ direction so that we can step through each pixel this required significant changes to the logic.

These changes were much larger than I anticipated when I started down this path however the advantage of sharing the same line drawing algorithm seems worth it to me. so I thought I try a pull request.

If this isn't acceptable I can spend some more time working through the problems in the original algorithm.

[spearson78]

I compared the original C code with the go code and finally tracked down the root cause.
The rendering of triangles is still a little strange (wide triangles at 12 and 6 o'clock and some dangling single pixels when rendering around 3 o'clock) these issues are however present in the original c code.

[spearson78]

Simpler fix was merged.