libs: encoder: h265: Adds VBR Encoding support
libs: encoder: h265: Adds VBR Encoding support

Enables Variable BitRate mode, which does set FrameRate and RateControl
parameters.

https://bugzilla.gnome.org/show_bug.cgi?id=778732

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