Do a real H.264 test decode to determine support

Firefox is buggy and reports support for H.264 but then throws errors
once we actually try to decode things. Detect this early by doing a
quick test decode of a single frame.

core/util/browser.js

@@ -9,6 +9,7 @@
  */
 
 import * as Log from './logging.js';
+import Base64 from '../base64.js';
 
 // Touch detection
 export let isTouchDevice = ('ontouchstart' in document.documentElement) ||
@@ -74,7 +75,7 @@ export let supportsWebCodecsH264Decode = false;
 
 async function _checkWebCodecsH264DecodeSupport() {
     if (!('VideoDecoder' in window)) {
-        return;
+        return false;
     }
 
     // We'll need to make do with some placeholders here
@@ -85,10 +86,56 @@ async function _checkWebCodecsH264DecodeSupport() {
         optimizeForLatency: true,
     };
 
-    const result = await VideoDecoder.isConfigSupported(config);
+    let support = await VideoDecoder.isConfigSupported(config);
-    supportsWebCodecsH264Decode = result.supported;
+    if (!support.supported) {
+        return false;
     }
-_checkWebCodecsH264DecodeSupport();
+
+    // Firefox incorrectly reports supports for H.264 under some
+    // circumstances, so we need to actually test a real frame
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1932392
+
+    const data = new Uint8Array(Base64.decode(
+        'AAAAAWdCwBTZnpuAgICgAAADACAAAAZB4oVNAAAAAWjJYyyAAAABBgX//4Hc' +
+        'Rem95tlIt5Ys2CDZI+7veDI2NCAtIGNvcmUgMTY0IHIzMTA4IDMxZTE5Zjkg' +
+        'LSBILjI2NC9NUEVHLTQgQVZDIGNvZGVjIC0gQ29weWxlZnQgMjAwMy0yMDIz' +
+        'IC0gaHR0cDovL3d3dy52aWRlb2xhbi5vcmcveDI2NC5odG1sIC0gb3B0aW9u' +
+        'czogY2FiYWM9MCByZWY9NSBkZWJsb2NrPTE6MDowIGFuYWx5c2U9MHgxOjB4' +
+        'MTExIG1lPWhleCBzdWJtZT04IHBzeT0xIHBzeV9yZD0xLjAwOjAuMDAgbWl4' +
+        'ZWRfcmVmPTEgbWVfcmFuZ2U9MTYgY2hyb21hX21lPTEgdHJlbGxpcz0yIDh4' +
+        'OGRjdD0wIGNxbT0wIGRlYWR6b25lPTIxLDExIGZhc3RfcHNraXA9MSBjaHJv' +
+        'bWFfcXBfb2Zmc2V0PS0yIHRocmVhZHM9MSBsb29rYWhlYWRfdGhyZWFkcz0x' +
+        'IHNsaWNlZF90aHJlYWRzPTAgbnI9MCBkZWNpbWF0ZT0xIGludGVybGFjZWQ9' +
+        'MCBibHVyYXlfY29tcGF0PTAgY29uc3RyYWluZWRfaW50cmE9MCBiZnJhbWVz' +
+        'PTAgd2VpZ2h0cD0wIGtleWludD1pbmZpbml0ZSBrZXlpbnRfbWluPTI1IHNj' +
+        'ZW5lY3V0PTQwIGludHJhX3JlZnJlc2g9MCByY19sb29rYWhlYWQ9NTAgcmM9' +
+        'YWJyIG1idHJlZT0xIGJpdHJhdGU9NDAwIHJhdGV0b2w9MS4wIHFjb21wPTAu' +
+        'NjAgcXBtaW49MCBxcG1heD02OSBxcHN0ZXA9NCBpcF9yYXRpbz0xLjQwIGFx' +
+        'PTE6MS4wMACAAAABZYiEBrxmKAAPVccAAS044AA5DRJMnkycJk4TPw=='));
+
+    let error = null;
+
+    let decoder = new VideoDecoder({
+        output: (frame) => {},
+        error: (e) => { error = e; },
+    });
+    let chunk = new EncodedVideoChunk({
+        timestamp: 0,
+        type: 'key',
+        data: data,
+    });
+
+    decoder.configure(config);
+    decoder.decode(chunk);
+    await decoder.flush();
+
+    if (error !== null) {
+        return false;
+    }
+
+    return true;
+}
+supportsWebCodecsH264Decode = await _checkWebCodecsH264DecodeSupport();
 
 /*
  * The functions for detection of platforms and browsers below are exported