Property collection functions were not really in sync which could result
in crashes in UI_LISTS.

The reason is that attributes of type color defined on domains other than
Points or Corners are still valid, but should really be skipped for the
special "color_attributes".
rna_AttributeGroup_color_length itself was fine here, it skips these,
but the iterator (rna_AttributeGroup_color_iterator_begin /
rna_Attributes_noncolor_layer_skip) wasnt, so when a UI_LIST
filter_items() would iterate the collection and would actually get
results were it shouldnt.

Now check a suiting domain in rna_Attributes_noncolor_layer_skip as
well.