Throttle frame callbacks to once per monitor refresh

Under some circumstances, the compositor can get into a commit-frame
callback busy loop with a client. For example, if a client redraws on
frame callbacks, but the resulting frame has empty damage (e.g. the
damaged part of the client is outside the monitor). Or if the client
simply commits with empty damage (looking at you, Firefox).

This behavior is compliant with the Wayland specification and with the
intended idea of frame callbacks, but causes a lot of unnecessary CPU
usage in the client and the compositor.

To solve this problem, this commit introduces frame callback throttling.

Every surface may only receive a single frame callback in one
monitor refresh cycle. If a surface commits resulting in no KMS frame
submission, a timer is created, that will fire at the predicted would-
be-VBlank time, and send the accumulated frame callbacks.

This way, a surface that redraws on frame callbacks will not notice
any change in frame callback delivery, if its commits suddenly stop
producing KMS updates.

src/layout.rs

@@ -579,11 +579,16 @@ impl<W: LayoutElement> MonitorSet<W> {
         }
     }
 
-    pub fn send_frame(&self, output: &Output, time: Duration) {
+    pub fn send_frame(
+        &self,
+        output: &Output,
+        time: Duration,
+        should_send: &impl Fn(&SurfaceData) -> bool,
+    ) {
         if let MonitorSet::Normal { monitors, .. } = self {
             for mon in monitors {
                 if &mon.output == output {
-                    mon.workspaces[mon.active_workspace_idx].send_frame(time);
+                    mon.workspaces[mon.active_workspace_idx].send_frame(time, should_send);
                 }
             }
         }
@@ -1826,10 +1831,12 @@ impl<W: LayoutElement> Workspace<W> {
         self.add_window(window, true);
     }
 
-    fn send_frame(&self, time: Duration) {
+    fn send_frame(&self, time: Duration, should_send: &impl Fn(&SurfaceData) -> bool) {
         let output = self.output.as_ref().unwrap();
         for win in self.windows() {
-            win.send_frame(output, time, None, |_, _| Some(output.clone()));
+            win.send_frame(output, time, None, |_, states| {
+                should_send(states).then(|| output.clone())
+            });
         }
     }