qt6.qtmultimedia: Compile ffmpeg multimedia plugin

This is now the default recommendation upstream for linux platforms

> https://doc.qt.io/qt-6.6/qtmultimedia-index.html#ffmpeg-as-the-default-backend
> In this release the FFmpeg framework is set as the default backend on
> Windows, macOS, Android, and Linux except Yocto distribution.

> The version shipped with Qt binary packages is FFmpeg 6.0
> and is tested by the maintainers.

libXrandr is required to compile support QT_WINDOW_CAPTURE_BACKEND=x11

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -53,3 +53,5 @@ The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been m
 
 - The `hardware.pulseaudio` module now sets permission of pulse user home directory to 755 when running in "systemWide" mode. It fixes [issue 114399](https://github.com/NixOS/nixpkgs/issues/114399).
 
+- QtMultimedia has changed its default backend to `QT_MEDIA_BACKEND=ffmpeg` (previously `gstreamer` on Linux or `darwin` on MacOS).
+  The previous native backends remain available but are now minimally maintained. Refer to [upstream documentation](https://doc.qt.io/qt-6/qtmultimedia-index.html#ffmpeg-as-the-default-backend) for further details about each platform.

pkgs/development/libraries/qt-6/modules/qtmultimedia.nix

@@ -13,8 +13,10 @@
 , gst-plugins-good
 , gst-libav
 , gst-vaapi
+, ffmpeg_6
 , libpulseaudio
 , wayland
+, libXrandr
 , elfutils
 , libunwind
 , orc
@@ -24,8 +26,8 @@
 qtModule {
   pname = "qtmultimedia";
   nativeBuildInputs = [ pkg-config ];
-  buildInputs = [ libunwind orc ]
-    ++ lib.optionals stdenv.isLinux [ libpulseaudio elfutils alsa-lib wayland ];
+  buildInputs = [ libunwind orc ffmpeg_6 ]
+    ++ lib.optionals stdenv.isLinux [ libpulseaudio elfutils alsa-lib wayland libXrandr ];
   propagatedBuildInputs = [ qtbase qtdeclarative qtsvg qtshadertools qtquick3d ]
     ++ lib.optionals stdenv.isLinux [ gstreamer gst-plugins-base gst-plugins-good gst-libav gst-vaapi ]
     ++ lib.optionals stdenv.isDarwin [ VideoToolbox ];