vaapidecode: calculate decoding latency
vaapidecode: calculate decoding latency

This is a naïve approach to the calculation of the VA-API decoding latency. It
takes into consideration when the frame-rate has some insane value.

https://bugzilla.gnome.org/show_bug.cgi?id=740419

gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi