Add demo/include/util/dither_matrix.h

This patch moves a repeatedly used dithering matrix from the scout
source code to a public header to share it across programs.
Remove nitpicker_gfx/miscmath.h
Move nitpicker_gfx/color.h to util/color.h
Remove nitpicker_gfx/nitpicker_types.h
Generalization of nitpicker's graphics backend

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/'.
Integrate scout widgets with Genode's Surface API