display driver to clean up its shit. Fixed X11 driver accordingly.

attached to it.

current canvas. This is useful to export dirty rectangles. Accordingly,
add caca_import_area_from_memory() and caca_import_area_from_file().

a more natural (x,y,w,h) 4-tuple to handle.

inner implementation still only handles one dirty rectangle, but this way
we can prepare supporting applictions for the future.

code for clarity.