Add trap_R_LerpTag and trap_R_ModelBounds to game

Dedicated server can either have model loading compiled into it (default behavior), or if SERVER_USE_RENDERER_DLOPEN=1 use external renderer from com_renderer.
Client uses whatever renderer it's using (internal or external) to load models for server.

If client reloads renderer (vid_restart, snd_restart, etc) all model handles become invalid, so GAME_VID_RESTART gets called so game vm can re-register models.