Commit dc069e42 removed the
assumption that 1-bpp PCX files are B&W, in favor of using the
provided palette, which is (supposedly?) the correct behavior.
However, there are evidently B&W files that do not specify a
palette, resulting in an all-black image (i.e., a 2-color indexed
image, whose both palette entries are black).  Since other
software, including older versions of GIMP, load such files
"correctly", let's fix this by falling back to a B&W palette when
the provded palette is uniform.