wayland: don't poll if there are no pending frames
wayland: don't poll if there are no pending frames

Otherwise the following poll may not return for an arbitrary amount of
time. This can happen if another wayland event queue has flushed and read
our events.

https://bugzilla.gnome.org/show_bug.cgi?id=795224

GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi