Draws inline XPM data.
This is a text-based 256-color image format designed for X11 and still very useful for making small icons,
since the data can be easily inserted into your source code.
FLTK recognizes a few extensions to the xpm color map:
Setting the number of colors negative means the second line in the array is a "compressed" colormap,
which is 4 bytes per color of character,r,g,b.
If all colors are grays and there is no transparent index,
it instead makes a MASK image,
where black draws the current color and white is transparent,
and all other grays are partially transparent.
This allows you to put antialiased glyphs into images.