When a small file is stored with a palette, a test is made to see if
removing the palette, and the overhead that goes with it, results in
an even smaller file.  That test was always attempting RGB or RGBA
mode even when the image in question was greyscale.  I added a check
for greyscale.

When checking if an image is smaller without a palette, use
the minimum bit depth for greyscale images instead of
defaulting to 8 bits.