libs: encoder: dummy context for get_surface_formats()
libs: encoder: dummy context for get_surface_formats()

Instead of creating (if it doesn't exist, yet) the encoder's context
the method gst_vaapi_encoder_get_surface_formats() now it creates
dummy contexts, unless the encoder has it already created.

The purpose of this is to avoid setting a encoder's context with a
wrong profile.

https://bugzilla.gnome.org/show_bug.cgi?id=771291

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