Add a funky dithered 16-colour mode to ewmhicon.

Add a funky dithered 16-colour mode to ewmhicon.

This adds a new option, --w32vga, which generates icons targeting the
16-colour "VGA" palette used by Windows.  This is the 8 combinations
of 0/max RGB values, plus the same set with the values halved, plus
a light gray.

This option enables a hand-coded pattern that approximates the desired
colours, and gives much more pleasing results in low colour modes.