NO_VR env var for monitor only mode

src/bin/vrtue.rs

@@ -8,15 +8,20 @@ extern crate gfx_device_gl;
 extern crate piston_window;
 
 use self::piston_window::{PistonWindow, Window, WindowSettings};
+use std::env;
 
 pub fn main() {
   env_logger::init().expect("env logger");
-  let mut vr = vr::VR::new().expect("VR init");
+  let mut vr = if env::var("NO_VR").is_ok() {
+    None
+  } else {
+    Some(vr::VR::new().expect("VR init"))
+  };
 
   let mut window: PistonWindow =
     WindowSettings::new("Hello, Britannia!", [1024; 2])
         .exit_on_esc(true)
-        .vsync(false)
+        .vsync(vr.is_none())  // Let VR throttle framerate, if available
         .build().expect("Building Window");
 
   let mut aux_command = window.factory.create_command_buffer();
@@ -44,7 +49,7 @@ pub fn main() {
     }
 
     // handle VR events
-    while let Some(ev) = vr.poll_next_event() {
+    while let Some(ev) = vr.as_mut().and_then(|vr| vr.poll_next_event()) {
       scene.event(Event::Vr(ev));
     }
   }