ewLoadImageXPM loads an image specified by filename.
It sets the width and height of the loaded image and returns
a newly allocated buffer containing color-indexed raster data.
The number_colors determines how many of the builtin colors to use,
generally 16, 8, or 4.
The image is converted into the colormap using Floyd-Steinburg
Error Diffusion.
ewLoadImageXPM returns NULL if not successful.
If remap is not NULL, the images colors are remapped accordingly.