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.

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- [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/script/dom/window.rs

@@ -1124,7 +1124,9 @@ impl Window {
         // When all these conditions are met, notify the constellation
         // that this pipeline is ready to write the image (from the script thread
         // perspective at least).
-        if (opts::get().output_file.is_some() || opts::get().exit_after_load) && for_display {
+        if (opts::get().output_file.is_some() ||
+            opts::get().exit_after_load ||
+            opts::get().webdriver_port.is_some()) && for_display {
             let document = self.Document();
 
             // Checks if the html element has reftest-wait attribute present.