avcodec: allow hardware acceleration plugin to use pic->opaque

(cherry picked from commit 3d29910e66214abb38d72a5d8172c4402144eb0d)

Conflicts:
	modules/codec/avcodec/video.c

modules/codec/avcodec/video.c

@@ -720,7 +720,7 @@ picture_t *DecodeVideo( decoder_t *p_dec, block_t **pp_block )
              * if needed */
             ffmpeg_CopyPicture( p_dec, p_pic, p_sys->p_ff_pic );
         }
-        else if( !p_sys->p_ff_pic->opaque )
+        else if( p_sys->p_va != NULL || p_sys->p_ff_pic->opaque == NULL )
         {
             /* Get a new picture */
             p_pic = ffmpeg_NewPictBuf( p_dec, p_context );