VAAPI-X11: Warn if both alpha and chroma keying are not supported for subtitles.

Some VAAPI backends do not support Alpha and chroma keying for subtitles. In that
case print a warning.

modules/codec/avcodec/vaapi_x11.c

@@ -507,6 +507,11 @@ int OpenVaapiX11(vlc_object_t *obj)
     vd->manage = Manage;
     vd->info = info;
 
+    /* */
+    if (!(sys->sflags & VA_SUBPICTURE_GLOBAL_ALPHA) &&
+        !(sys->sflags & VA_SUBPICTURE_CHROMA_KEYING))
+        msg_Warn(vd, "Display server does not support alpha or chroma for subtitles.");
+
     /* */
     bool is_fullscreen = vd->cfg->is_fullscreen;
     if (is_fullscreen && vout_window_SetFullScreen(sys->embed, true))