fix ffmpeg vaapi decoding issue

change the flatpak manifest to include the ffmpeg extension

flatpak/io.github.zen_browser.zen.yml.template

@@ -4,6 +4,11 @@ runtime-version: '23.08'
 sdk: org.freedesktop.Sdk
 base: org.mozilla.firefox.BaseApp
 base-version: '23.08'
+add-extensions:
+  org.freedesktop.Platform.ffmpeg-full:
+    directory: lib/ffmpeg
+    version: 23.08
+    add-ld-path: .
 command: launch-script.sh
 finish-args:
   - --share=ipc
@@ -27,6 +32,7 @@ modules:
     buildsystem: simple
     build-commands:
       - mv zen /app/
+      - mkdir -p /app/lib/ffmpeg
 
       - install -Dm0755 metadata/launch-script.sh ${{FLATPAK_DEST}}/bin/launch-script.sh
       - install -Dm0644 metadata/policies.json ${{FLATPAK_DEST}}/bin/distribution/policies.json
@@ -44,4 +50,4 @@ modules:
         url: https://github.com/zen-browser/flatpak/releases/download/{version}/archive.tar
         sha256: {flatpak_sha256}
         strip-components: 0
-        dest: metadata
+        dest: metadata