Portability notes:
- At the moment none of this is done as object-oriented but it's
deliberately coded so that converting it to OO should be fairly
trivial.  Eventually this WILL be done but Xanni wants it to wait
until after at least the first stable release.

- The display_* (physical screen) and draw_* (logical screens)
functions are the ONLY ones that deal with display (the view_* modules
are more abstract as the raster/spin/flip ideas are relavent to all
displays so we don't include those).  Eventually we should modularize
them so we can pull them out and 'slot' another module in for a
completely different look.  The 'interface' between the display/window
functions and the rest of the program is via these functions only:
   display_open()
   display_close()
   display_resize()
   display_refresh()
   display_clear()
   draw()
   draw_status()   (NB: Not yet implemented)
None of the other display_* or draw_* functions should be called
from elsewhere in the program.

- The input_* functions should similarly be separated as alternative
input methods (eg: mouse) will then be easy to slot in.  Interface
functions are:
   input_get_any()        (polls for input, returns undef if no keys waiting)
   input_get_direction()  (waits until it gets a valid direction keystroke)