Fix for Bug 159947 - saving 1bpp PCX files

+Allows user to save indexed 1bpp and 4bpp PCX files automatically,
depending on the number of colors used.

+Creates a general function with parameters for both bpp and # of
planes, to handle more PCX file types (1bpp, 2planes; 1bpp, 2planes;
4bpp, 1plane).

+Removes assumption when loading colormap that 1bpp files are black &
white only; loads both colors from PCX palette header instead.

https://bugzilla.gnome.org/show_bug.cgi?id=159947