Set the "category" bitfield for the given geom. This bitfield is used by spaces to govern which geoms will interact with each other. The bitfield is guaranteed to be at least 32 bits wide. The default category and collide values for newly created geoms have all bits set. Note this is NOT CLS-compliant (due to the use of ulong to hold the 32-bit bitfield) TODO: Implement a CLS-compliant work-around or justify why not