Make gst_vaapi_decoder_get_codec_state() return the original codec state,
i.e. make the GstVaapiDecoder object own the return state so that callers
that want an extra reference to it would just gst_video_codec_state_ref()
it before usage. This aligns the behaviour with what we had before with
gst_vaapi_decoder_get_caps().

This is an ABI incompatible change, library major version was bumped from
previous release (0.5.2).