Changed FogData to png from Point[]

Png has a more consistent filesize, and after 1 session we had reached
300KB (god knows how many actual Point values).
Forced repaint when drawing fog

Otherwise, large maps end up not drawing the new fog area fast enough.
Performance was iffy so toggled the HQ flag as well. Will look into
performance again later.
Added save fog on exit

on dispose, actually, to handle the form being forcibly shut.