allow reregistering but check for differences

allow reregistering but check for differences

babl_model_new() emitted a warning when reregistering a model,
babl_format_new(), babl_type_new(), and babl_component_new()
quietly returned a preexisting object.

Changed so that all four functions return the preexisting object
if the new registration doesn't conflict, otherwise make the
error fatal.

(Fixed some conflicting and redundant registrations for
"Y'CbCr u8" along the way)