Set a minimum Tiles window size of 800x480 when not using touch UI

Resizing the window too small can cause Tiles crashes. We might
consider allowing a smaller X limit. There is existing code to prevent
rc tiles options from using smaller than 800x480, so I kept the window
limits consistent with that.
Fix resizing of the Tiles window

The SDL event loop forced a redraw to display the current mouse cell's
alt. text, but these redraws were occurring not only when the mouse
moved. Moved this logic into the mouse movement handling section and
did some other refactoring.