VAAPI: Only accept VAAPI type of pictures when vaapi-xcb is active.

Make sure that when the VAAPI-XCB video output has been started no other picture chroma,
will be accepted. The punishment for not doing this is random crashes, since surfaces are
shared between display pictures and the decoding thread. The picture is the owner of the
surface. If not honoring this, then the decoder might write into surfaces being displayed.
This will result in random crashes since surface memory will be randomly overwritten.

modules/codec/avcodec/va.h

@@ -30,6 +30,8 @@ struct vlc_va_t {
     char *description;
     vlc_object_t *obj;
 
+    bool b_direct;
+
     int  (*setup)(vlc_va_t *, void **hw, vlc_fourcc_t *output,
                   int width, int height);
     int  (*get)(vlc_va_t *, AVFrame *frame);

modules/codec/avcodec/video.c

@@ -996,10 +996,15 @@ static int ffmpeg_GetFrameBuf( struct AVCodecContext *p_context,
     p_ff_pic->opaque = NULL;
 
     if( p_sys->p_va )
+    {
+        do
         {
             p_pic = ffmpeg_NewPictBuf( p_dec, p_sys->p_context );
+        } while( p_sys->p_va->b_direct && p_pic == NULL );
+
         if( p_pic && p_pic->format.i_chroma == VLC_CODEC_VAAPI_SURFACE )
         {
+            p_sys->p_va->b_direct = true;
             p_ff_pic->opaque = (void*)p_pic;
             p_ff_pic->type = FF_BUFFER_TYPE_USER;
 #if LIBAVCODEC_VERSION_MAJOR < 54
@@ -1008,7 +1013,7 @@ static int ffmpeg_GetFrameBuf( struct AVCodecContext *p_context,
 #endif
             vlc_va_Put( p_sys->p_va, p_ff_pic, p_pic );
         }
-        else
+        else if( !p_sys->p_va->b_direct )
         {
             if( p_pic ) picture_Release( p_pic );
 #ifdef HAVE_AVCODEC_VA