ability to run 2D physics in a thread

also, 2D physics is now thread safe too.
see physics_2d/thread_model

2d/platformer/engine.cfg

@@ -28,6 +28,7 @@ spawn=[key(F1), jbutton(0, 11)]
 [physics_2d]
 
 default_gravity=700
+thread_model=1
 
 [rasterizer]