If a character had a bottom offset for one view but none for another,
the offset was not getting cleared properly when switching

The default displayed unit was cm^2, but it was m^2 internally -
so it would wind up shrinking if you went to m^2. This needs to be
fixed more generally, though - it should be able to start at any unit
and still work properly.

units

These let a character have micro/normal/macro heights

This was causing things to vanish when editing numbers

It should put the height in the currently chosen unit into the input box

For whatever reason, I cannot seem to draw an SVG to a canvas.
Therefore, the selection test function does not work.
The current solution is to just skip the logic in Firefox and
use the standard bounding-box selection.

Clicks that hit the bounding box of any entity were unable to deselect, since
the canvas was not listening for mousedown events. It now is.

The clicked entity is rendered to a canvas and tested for opacity at that point.
Very large entities caused a very large canvas to be created.
Now, the canvas is limited to 4000x4000 in size