Missile velocity fix

build_release.bat

@@ -1,2 +1,3 @@
+call shadergen.bat
 set compileopts=/Fe"flight_release" /O2
 call build_msvc.bat

gamestate.c

@@ -206,13 +206,13 @@ void box_remove_from_boxes(GameState *gs, Entity *box)
 V2 player_vel(GameState *gs, Entity *e);
 V2 entity_vel(GameState *gs, Entity *e)
 {
-  assert(e->is_box || e->is_player || e->is_grid || e->is_explosion);
+  assert(e->is_box || e->is_player || e->body != NULL || e->is_explosion);
   if (e->is_box)
     return box_vel(e);
   if (e->is_player)
     return player_vel(gs, e);
-  if (e->is_grid)
+  if (e->body != NULL)
-    return grid_vel(e);
+    return cp_to_v2(cpBodyGetVelocity(e->body));
   if (e->is_explosion)
     return e->explosion_vel;
   assert(false);