codec/avcodec/vaapi.c: double the amount of surfaces requested

In case vaapi-x11 video output is used, then the default amount of surfaces is
not enough to guarantee smooth playback. Therefor double the amount of requested
surfaces. Currently there is no easy way of determining if vaapi-x11 video output
is going to be used, because the vaapi-codec is most likely to be constructed first.

modules/codec/avcodec/vaapi.c

@@ -113,6 +113,8 @@ static int Open( vlc_va_vaapi_t *p_va, int i_codec_id )
     default:
         return VLC_EGENERIC;
     }
+    /* FIXME: in case of vaapi-x11 double the number of surfaces */
+    i_surface_count *= 2;
 
     /* */
     memset( p_va, 0, sizeof(*p_va) );