Add a software triangle rasterizer

[nical]

This would be very useful for reftests, and as a fallback when graphics drivers aren't working well.
It would also be interesting to see how close to a classic CPU path rasterizer we can get with something like this.
There's an empty function waiting to be implemented in extra/src/triangle_rasterizer.rs.

[nical]

This was implemented though some tests need to be written with it.