video: Always pause if media-stream changed

When you have a `GtkVideo` that's already playing some file, then
using `gtk_video_set_file()` (or `gtk_video_set_media_stream()`) will
_not_ pause the old media stream if "autoplay" is not set.

What happens, is that you then get a `GtkVideo` which is playing both
sound from the old stream, as well as video and sound from the new one,
which is quite confusing.

when setting a new media-stream, we should just _always_ put the
previous one to paused to prevent any confusion.

gtk/gtkvideo.c

@@ -615,8 +615,7 @@ gtk_video_set_media_stream (GtkVideo       *self,
 
   if (self->media_stream)
     {
-      if (self->autoplay)
-        gtk_media_stream_pause (self->media_stream);
+      gtk_media_stream_pause (self->media_stream);
       g_signal_handlers_disconnect_by_func (self->media_stream,
                                             gtk_video_notify_cb,
                                             self);