Experimental: depth testing

[colinrubow]

Overview: What does this pull request change?

Sorts mobjects into depth_testing and non-depth_testing. Performs proper depth testing with transparent mobjects as well.

Motivation and Explanation: Why and how do your changes improve the library?

To properly perform depth testing with transparent objects, you first render opaque objects first. Then you sort the transparent mobjects and render them from back to front without depth_buffer writing. This implementation then renders the non-depth testing mobjects on top of all the depth-tested mobjects.

Links to added or changed documentation pages

NA

Further Information and Comments

NA

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested