[advanced] movegravity() and movement vectors

[voron00]

A 'reversed' movegravity function in case anyone wants to 'mess around with' in libcod:

		void gsc_entity_movegravity(scr_entref_t id)
{
    vec3_t dir;
    float time;

    if ( ! stackGetParams("vf", &dir, &time))
    {
        stackError("gsc_entity_movegravity() one or more arguments is undefined or has a wrong type");
        stackPushUndefined();
        return;
    }

    gentity_t *entity = &g_entities[id];

    entity->s.pos.trType = TR_GRAVITY;
    entity->s.pos.trTime = level.time;
    entity->s.pos.trDuration = time * 1000.0;

    VectorCopy(entity->r.currentOrigin, entity->s.pos.trBase);
    VectorCopy(dir, entity->s.pos.trDelta);

    BG_EvaluateTrajectory(&entity->s.pos, level.time, entity->r.currentOrigin);
    SV_LinkEntity(entity);

    stackPushBool(qtrue);
}