Rather than creating new geometric primitives on the heap, wouldn't it be more flexible if the caller can provide the memory to initialize the structure in?

Then instead of

  struct tg_geom *geom = tg_geom_new_point(-112, 33);
  if (!geom) {
      // System is out of memory.
  }

you could do

  struct tg_geom geom;
  tg_geom_init_point(&geom, -112, 33);

without need for error checking or cleanup. You can still allocate on the heap if you want but you wouldn't have to.