-Coordinates obtained from Selector.pickTile() are updated to be in the
correct relative system. So a coordinate of something like (42.0,42.0)
is converted to (2.0,2.0). This is with a GRID_WDITH = 12 though, but
the system works for any GRID_WIDTH;
-Coordinates obtained from not hitting the map are always (-1.0,-1.0) so
use the to check if the map is not currently under the mouse.
-Note that these changes occur AFTER the coordinate is returned by the
Selector. You can see the small bit of code that does the work inside
the mousedown handler in GameCanvas.