lavc: Add new VDA hwaccel

It leverages the new hwaccel 1.2 features:

- get_buffer2 is never called
- the internal context is automatically initialized/deinitialized

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/allcodecs.c

@@ -79,6 +79,7 @@ void avcodec_register_all(void)
     REGISTER_HWACCEL(H264_DXVA2,        h264_dxva2);
     REGISTER_HWACCEL(H264_VAAPI,        h264_vaapi);
     REGISTER_HWACCEL(H264_VDA,          h264_vda);
+    REGISTER_HWACCEL(H264_VDA_OLD,      h264_vda_old);
     REGISTER_HWACCEL(H264_VDPAU,        h264_vdpau);
     REGISTER_HWACCEL(MPEG1_VDPAU,       mpeg1_vdpau);
     REGISTER_HWACCEL(MPEG2_DXVA2,       mpeg2_dxva2);