Update Polygons' bounding box calculations

[anderoonies]

Resolves #119

Summary

This updates the way Polygon's bounding boxes are calculated to account for the possibility of negative points. Additionally, when drawing bounding boxes in debug mode, draw the absolute bounds rather than a square with the width/height of the AABB around the x,y position to account for the possibility of negative points.

Changes:

• Polygon now implements _updateBounds to manually calculate its bounding box
• Thing's draw method now will draw a debug bounding box with no context translation at the bounds' position. This is to handle negative coordinates (relative to the shapes' x, y position) in the bounding box.

Screenshots of the change:

Checklist

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