decoder: use GstVideoCodecState.

decoder: use GstVideoCodecState.

Use standard GstVideoCodecState throughout GstVaapiDecoder and expose
it with a new gst_vaapi_decoder_get_codec_state() function. This makes
it possible to drop picture size (width, height) information, framerate
(fps_n, fps_d) information, pixel aspect ratio (par_n, par_d) information,
and interlace mode (is_interlaced field).

This is a new API with backwards compatibility maintained. In particular,
gst_vaapi_decoder_get_caps() is still available.

Signed-off-by: Sreerenj Balachandran <sreerenj.balachandran@intel.com>
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>