#85 1-, 2-, and 4-bit greyscale modes ignored

[Viatrix]

Expected behaviour: optipng chooses the 1-bit, 2-bit, or 4-bit greyscale colour type when applicable.
Actual behaviour: optipng chooses the indexed-colour type in such cases.
(1-bit, 2-bit, or 4-bit greyscale images are smaller than the equivalent palette-using images, as the PLTE chunk is not present)