Auto merge of #13827 - jdm:screenshot, r=Ms2ger

Support taking screenshots when a webdriver server is active.

Improve the experience of using webdriver to take screenshots of Servo. When a screenshot can't be obtained immediately, increase the delay before trying again from 2ms to 1s.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13826
- [X] These changes do not require tests because we can't test webdriver yet.

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components/compositing/compositor.rs

@@ -578,6 +578,9 @@ impl<Window: WindowMethods> IOCompositor<Window> {
 
             (Msg::CreatePng(reply), ShutdownState::NotShuttingDown) => {
                 let res = self.composite_specific_target(CompositeTarget::WindowAndPng);
+                if let Err(ref e) = res {
+                    info!("Error retrieving PNG: {:?}", e);
+                }
                 let img = res.unwrap_or(None);
                 if let Err(e) = reply.send(img) {
                     warn!("Sending reply to create png failed ({}).", e);