bullet: release GIL during expensive do_physics call

Fixes #726

panda/src/bullet/bulletWorld.h

@@ -58,7 +58,7 @@ PUBLISHED:
   void set_gravity(PN_stdfloat gx, PN_stdfloat gy, PN_stdfloat gz);
   const LVector3 get_gravity() const;
 
-  int do_physics(PN_stdfloat dt, int max_substeps=1, PN_stdfloat stepsize=1.0f/60.0f);
+  BLOCKING int do_physics(PN_stdfloat dt, int max_substeps=1, PN_stdfloat stepsize=1.0f/60.0f);
 
   BulletSoftBodyWorldInfo get_world_info();