added Vector3::operator-=()

completes (arithmetic) compound assignments for vec3
changed geometry bounding box size

bounding box enlarged by fixed amount instead of scaling
removed dead code

removed dead functions Billboard_FaceCamera(), Billboard_FaceViewingPlane()
polymesh: differentiate rebuilds, updates

rebuild when topology changed (eg. new_edge, del_node),
update when geometry changed (eg. set_position, set_color)
show edgeRenderer.SetTransform in rendermetrics
improved performance of line edges

improved performance by offloading work to gpu