Verify WhiteIsZero invert, add Gray(1) color type

[197g]

This was found through the fax4 decoding that is always bilevel.

No longer silently ignore an unknown color type when a photometric interpretation requires an inversion. DICOM requires that WhiteIsZero and BlackIsZero are only used for samples-per-pixel=1 but we do not strictly enforce that early. Previously for the result Multiband color we just ignored the tag which is also not right.

Closes: #296

@Shnatsel the image you linked is now in the test set and image's convert example gives the correct colors in a test. I'm still puzzled about the code block in fax where bits seem flipped, to me at least. Maybe fax flipped their Black/White colors in the returned stream, too? It's puzzling.

[Shnatsel]

Well, it fixes decoding of imagemagick output (which you added to the repo) and the libtiff fax4 test file. And the BlackIsZero image committed originally still works. So good enough for me.

[Shnatsel]

There is a non-fax-encoded 1-bit TIFF in the image test suite, and with this change it still decodes fine: https://github.com/image-rs/image/blob/main/tests/images/tiff/testsuite/l1.tiff

It is in BlackIsZero mode

[197g]

I've verified with libtiff. It seems the old 'convention' or default behavior of treating whiteiszero might have eaten its tail in here somehwere. Here's the runlength decoding loop for libtiff: https://gitlab.com/libtiff/libtiff/-/blob/85f2ac8e0b01cb7db2bbecf4a3b891bdbef67938/libtiff/tif_fax3.c (_TIFFFax3fillruns)

The critical portion is that its loop always does white-black at the same time, fax starts with white. And in that loop we have:

// white section
                        for (; n && !isAligned(cp, int64_t); n--)
                            *cp++ = 0x00;
[…]
// black section
                        for (; n && !isAligned(cp, int64_t); n--)
                            *cp++ = 0xff;

Whatever history is behind here it is consistent with treating the run that is, per ITU, white as 0 and black as 1. I'll add comments and we can merge this.