Detect broken Firefox H.264 decoder

The Firefox H.264 decoder on Windows might simply just refuse to deliver
any finished frames. It also doesn't deliver any errors.

Detect this early by expecting a frame after flush() has completed.

core/util/browser.js

@@ -113,10 +113,11 @@ async function _checkWebCodecsH264DecodeSupport() {
         'NjAgcXBtaW49MCBxcG1heD02OSBxcHN0ZXA9NCBpcF9yYXRpbz0xLjQwIGFx' +
         'PTE6MS4wMACAAAABZYiEBrxmKAAPVccAAS044AA5DRJMnkycJk4TPw=='));
 
+    let gotframe = false;
     let error = null;
 
     let decoder = new VideoDecoder({
-        output: (frame) => {},
+        output: (frame) => { gotframe = true; },
         error: (e) => { error = e; },
     });
     let chunk = new EncodedVideoChunk({
@@ -135,6 +136,13 @@ async function _checkWebCodecsH264DecodeSupport() {
         error = e;
     }
 
+    // Firefox fails to deliver the error on Windows, so we need to
+    // check if we got a frame instead
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1932579
+    if (!gotframe) {
+        return false;
+    }
+
     if (error !== null) {
         return false;
     }