src/gfxcore.cc: Invalidate the hit-test grid when surface or
underground legs are toggled on or off.
src/gfxcore.cc: Divide evenly over the full number of hittest grid
boxes, rather than not really using the final box in each direction.
src/gfxcore.cc: Increase the threshold for how close the pointer
needs to be to a station from 5 pixels to 7 to try to help
touchscreen users.  Reported by Hugh St Lawrence.
src/gfxcore.cc,src/gfxcore.h,src/guicontrol.cc: Add "hit test debug"
mode, which shows the hit test grid and how many entries are in each
box (toggled by F3).