emcc frontend: stop indiscriminately squashing mouseups.

emcc frontend: stop indiscriminately squashing mouseups.

The mouseup listener was calling event.preventDefault(), as part of
the mechanism for making mouse clicks and drags on the puzzle's resize
handle have resizing effects _instead_ of the normal browser
behaviour. However, calling event.preventDefault() on _every_ mouseup,
rather than just the ones associated with the resize handle, was
overkill, and I've recently noticed that it's breaking attempts to
select from the game type dropdown by clicking the mouse. So now I'm
only calling preventDefault() on the mouseups that I have reason to
think are actually relevant to what I'm trying to do.

(I don't know why I've only just noticed this. I suppose a change of
behaviour between Firefox versions is the most likely cause.)