Don't perform composite unless renderer has indicated it's idle

The rendergl::render_scene() function is being called more often than
it needs to be, sometimes before the renderer has finished.  This
could result in PNG images that are drawn before the frame trees set
in SetIds is rendered.

src/components/main/compositing/run.rs

@@ -23,6 +23,7 @@ use layers::rendergl;
 use layers::scene::Scene;
 use opengles::gl2;
 use png;
+use servo_msg::compositor_msg::IdleRenderState;
 use servo_msg::constellation_msg::{ConstellationChan, NavigateMsg, ResizedWindowMsg, LoadUrlMsg};
 use servo_msg::constellation_msg;
 use servo_util::time::profile;
@@ -49,6 +50,7 @@ pub fn run_compositor(compositor: &CompositorTask) {
     let mut window_size = Size2D(window_size.width as uint, window_size.height as uint);
     let mut done = false;
     let mut recomposite = false;
+    let mut composite_ready = false;
     let graphics_context = CompositorTask::create_graphics_context();
 
     // Keeps track of the current zoom factor
@@ -82,7 +84,13 @@ pub fn run_compositor(compositor: &CompositorTask) {
                 Exit => done = true,
 
                 ChangeReadyState(ready_state) => window.set_ready_state(ready_state),
-                ChangeRenderState(render_state) => window.set_render_state(render_state),
+                ChangeRenderState(render_state) => {
+                    window.set_render_state(render_state);
+                    composite_ready = match render_state {
+                        IdleRenderState => true,
+                        _ => false,
+                    }
+                }
 
                 SetUnRenderedColor(_id, color) => {
                     match compositor_layer {
@@ -404,7 +412,7 @@ pub fn run_compositor(compositor: &CompositorTask) {
         // Check for messages coming from the windowing system.
         check_for_window_messages(window.recv());
 
-        if recomposite {
+        if recomposite && composite_ready {
             recomposite = false;
             composite();
         }