NO_VR env var for monitor only mode

src/scenes/world.rs

@@ -201,7 +201,7 @@ impl<D: gfx::Device,
   }
 
   fn update(&mut self,
-            vr: &mut vr::VR,
+            vr: &mut Option<vr::VR>,
             encoder: &mut gfx::Encoder<D::Resources, D::CommandBuffer>) {
     const NANOS_PER_MILLI: u32 = 1_000_000;
     const MILLIS_PER_SEC: u64 = 1_000;
@@ -210,7 +210,7 @@ impl<D: gfx::Device,
 
     for (pad, track) in self.pads.iter_mut() {
       let mode = track.0;
-      if let Some(state) = vr.get_controller_state(*pad) {
+      if let Some(state) = vr.as_ref().and_then(|vr| vr.get_controller_state(*pad)) {
         if let Some(old_state) = track.1 {
           match mode {
             TrackMode::Touch => {