Add a headless null compositor

We also have to disable rendering in headless mode because it uses OpenGL calls
for now.

Fixes #471.

src/components/gfx/render_task.rs

@@ -204,6 +204,13 @@ impl<C: RenderListener + Send,T:Send+Freeze> RenderTask<C,T> {
     }
 
     fn render(&mut self, tiles: ~[BufferRequest], scale: f32) {
+        // In headless mode, disable the renderer, because it makes OpenGL
+        // calls.  Once we have CPU rendering we should render in CPU mode and
+        // just disable texture upload.
+        if self.opts.headless {
+            return;
+        }
+
         let render_layer;
         match self.render_layer {
             Some(ref r_layer) => {