Added a polygon scanline rendering algorithm.

It's a lot faster than barycentric interpolation, as it only needs to
compute the depth once - and can then use basic interpolation to render
lines.
The obstruction filter can now rapidly check if a polygon is in front of
any visible points, using a simple comparison operation.
Even though it might check more points, it iterates through polygons
instead of points, and large polygons fail fast.

Additional improvements include using a simple depth check (avoid
calculating rays which might be error-prone), and allowing future
improvements like comparing polygons with each other.

Memory usage should also be a lot more stable.

It also improves speed of rendering of meshes into PNG images.