Create Render2, analogous to Render3 and add new SDF2 renderer.

[yeicor]

This pull request creates the Render2 interface, very similar to Render3 (#41), implementing it for marching squares (uniform and quadtree).

It also adds a new algorithm that I've been developing capable of quickly creating simplified and sharp shapes for SDF2s (e.g. a rotated square with only 4 lines), and hopefully do the same for SDF3s in the future (help wanted). It is implemented in a different repo but it adds no other new dependencies to this project. See the repo for examples.