Fix re-sorting to prevent single-frame blips of incorrect sort

[anderoonies]

Summary

As a performance optimization, re-sorting the element pool as a result of a layer change or element removal was performed at the same time as a redraw. This could result in a single frame with out-of-order graphics.
This library doesn't aim to be the most performant, and keeping with expectations is better, so resorting will now happen prior to drawing if 1) an element has been added 2) an element has been removed 3) an element has had its layer changed.

Changes:

• Keep track of sort invalidation on Graphics manager, which is updated when an element is added
• Sort the element pool before drawing when necessary
• Tests

Checklist

• npm test passes
• Unit tests are included / updated
• Documentation has been updated where relevant