Add missing introspection annotations

[peterholzer]

Hi, in the .gir file, the function gthree_geometry_add_attribute and others are marked introspectable="0". This seems to be the case for more functions ingthreegeometry.h and in other headers.

E.g. gthree_geometry_add_morph_attribute is marked introspectable, so I think to fix gthree_geometry_add_attribute, an annotation for the return value is needed.

BR Peter

[alexlarsson]

I don't really use the annotations, but if you (or someone else) does I'd love patches for them.