Also switches from setting an invalid attribute to using
the attribute dataset to store the old unit for the unit
selectors and defers loading a scene until everything
is configured.

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).

Entities positioned above the ground were going off the screen.

There are probably still problems in the x-direection.