libs: context: split context_create()
libs: context: split context_create()

Split the funcion context_create() into context_create() and
config_create().

Decoupling VAConfig and VAContext during context creation, we could
query the VAConfig for the supported surface's formats without creating
a VAContext.

https://bugzilla.gnome.org/show_bug.cgi?id=769266

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