Panning does not respect cursor start position

[reinderien]

When you middle-click to pan, the board center position jumps. It seems that the panning uses the current mouse position to place the original center of the board, not to place the position upon which the user clicked.

[ewtoombs]

It was mathematically the easiest way of getting the panning working right away. I have a fix for it in my notebook somewhere which will basically work, but it will behave more and more weirdly the further away from the centre you are. Right now, the background rendering isn't infinite yet, so it's unlikely the weirdness will properly manifest at the moment. So it makes sense to delay this until after I have infinite background rendering implemented. That will take some mathing. Still, useful to have this as an official issue.