gtk: When setting format check if pending format changed
gtk: When setting format check if pending format changed

In case the format changed fast and the pending format is different
than the currently set but the currently set is equal to the pending
one we could end up having mismatch between the finally set format
and the data stream format.

https://bugzilla.gnome.org/show_bug.cgi?id=755542

GStreamer/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good