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.

tests/decode_images.rs

@@ -678,3 +678,9 @@ fn bytes_gray_f32() {
 fn test_fax4() {
     test_image_sum_u8("fax4.tiff", ColorType::Gray(1), 7802706);
 }
+
+#[test]
+#[cfg(feature = "fax")]
+fn test_fax4_white_is_min() {
+    test_image_sum_u8("imagemagick_group4.tiff", ColorType::Gray(1), 3742820);
+}