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).

Very large entities are confusing to add - they appear for a brief moment
and then grow off the screen. This shows an arrow if an entity is at least
ten times taller than the viewport.