Verify WhiteIsZero invert, add Gray(1) color type

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.

Cargo.toml

@@ -21,7 +21,7 @@ half = { version = "2.4.1" }
 quick-error = "2.0.1"
 
 # Rename to avoid confusion with the feature name.
-fax34 = { package = "fax", version = "0.2.4", optional = true }
+fax34 = { package = "fax", version = "0.2.6", optional = true }
 flate2 = { version = "1.0.20", optional = true }
 weezl = { version = "0.1.10", optional = true }
 zstd = { version = "0.13", optional = true }