Various clean-ups to improve consistency and readability: drop unused
macro definitions, drop initialization of vars that are zero-initialized
from the base class, drop un-necessary casts.
Add gst_vaapi_encoder_set_keyframe_period() interface to allow the
user control the maximum distance between two keyframes. This new
property can only be set prior to gst_vaapi_encoder_set_codec_state().
A value of zero for "keyframe-period" gets it re-evaluated to the
actual framerate during encoder reconfiguration.
Add interface to communicate configurable properties to the encoder.
This covers both the common ones (rate-control, bitrate), and the
codec specific properties.
https://bugzilla.gnome.org/show_bug.cgi?id=719529
