Entities derived from models can now have volumes

This also updates the model-processing script to automatically
insert the model data into the appropriate source file.

These use a new entity-maker.

Firefox won't draw SVGs to a canvas correctly without width/height
attributes (even if a viewBox is present). This commit inserts those
values where needed and removes the Firefox check from the click
detection code, so it works properly (and screenshots are fixed, too).

units