libs: encoder: caps can change at any time
libs: encoder: caps can change at any time

The encoder should be able to change its caps even it is already
processing a stream.

This is suppose to happen after a flush so the codedbuf_queue should
be empty.

https://bugzilla.gnome.org/show_bug.cgi?id=775490

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