libtas: Add withForceX11 option

libTAS needs to be run under X11 to capture inputs. Since libTAS is a QT app, it can launch in Wayland though. Thus, add an override option (enabled by default) which passes a `QT_QPA_PLATFORM=xcb` environment variable to force QT to run on X11.

pkgs/by-name/li/libtas/package.nix

···
   file,
   binutils,
   makeDesktopItem,
+
+  # Forces libTAS to run in X11.
+  # Enabled by default because libTAS does not support Wayland.
+  withForceX11 ? true,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
···
           ffmpeg.lib
         ]
       } \
+      ${lib.optionalString withForceX11 "--set QT_QPA_PLATFORM xcb"} \
       --set-default LIBTAS_SO_PATH $out/lib/libtas.so
   '';