This patch re-arranges nitpicker's graphics backend in a more modular
and expandable way. Generalized versions of the 'Canvas',
'Chunky_canvas', and 'Pixel_*' classes have been moved to
'os/include/util/' and 'os/include/os'. The only remaining parts that
are specific to nitpicker's needs are a few drawing functions, each
located in a distinct header at 'os/include/nitpicker_gfx/'.
This patch makes nitpicker's geometry utilities available for the use
by other programs. Thereby, the 'Point', 'Area', and 'Rect' classes
have become templates that take the coordinate type and distance type
as arguments.
