codec/avcodec/vaapi_x11.c: Center position of subpictures.

Center position of subpictures which looks better.

modules/codec/avcodec/vaapi_x11.c

@@ -425,7 +425,7 @@ static VASubpictureID SubpictureCreate(vout_display_t *vd, vlc_va_surface_t *sur
                                    /* src rectangle */
                                    src_left, src_top, src_right, src_bottom,
                                    /* dest rectangle */
-                                   dst_left, dst_top, dst_right - dst_left, dst_bottom - dst_top,
+                                   ((video.width - dst_right) / 2), dst_top, dst_right - dst_left, dst_bottom - dst_top,
                                    flags);
     if (status != VA_STATUS_SUCCESS)
     {