VAAPI-XCB: require option --ffmpeg-hw to be set

Testing for option '--ffmpeg-hw' to be enabled makes it possible to gracefully
failover to another video output (only if the vout has not been forced, aka: '-V'
option).

modules/codec/avcodec/vaapi_x11.c

@@ -205,6 +205,13 @@ int OpenVaapiX11(vlc_object_t *obj)
 {
     vout_display_t *vd = (vout_display_t *) obj;
 
+    const bool b_use_hw = var_GetBool( obj->p_libvlc, "ffmpeg-hw" );
+    if (!b_use_hw)
+    {
+        msg_Warn( obj, "VAAPI-XCB requires GPU hardware decoding to be enabled (option --ffmpeg-hw)" );
+        return VLC_EGENERIC;
+    }
+
     if (!vlc_xlib_init(obj))
         return VLC_EGENERIC;