Fix GUI scaling to avoid gaps around GUI when width isn't a multiple of scaling #177

At the moment, the GUI constraint solving code (for calculating GUI layout) just always works at 1 times scale and the result gets scaled up by a factor. This means that if the screensize isn't a multiple of the GUI scale then there's a gap on the bottom and left of the GUI (i.e. the hotbar will be a pixel from the bottom of the screen, and an overlay won't extend all the way to the bottom right corner).

To fix this, the constraints system will probably have to be scaling-aware.