Board can be flipped inside out

[reinderien]

When you middle-pan outside of the circle, the board turns inside out. There should be a constraint that mouse movement outside of the circle is ignored.

[ewtoombs]

I thought about fixing this, but I kind of like it ;)
But seriously, though, when I get infinite panning and background rendering working properly, the inside outness is seriously going to break things, since it will briefly pan the infiniboard to infinity and back. So this will have to get addressed as part of the infinite rendering and panning work.

[Fierthraix]

#ItsAFeatureNotABug