lavu/hwcontext_vaapi: sync surface before export its DRM handle

According to description of vaExportSurfaceHandle in libva, vaSyncSurface
must be called if the contents of the surface will be read.

Fixes ticket #9967.
Reviewed-by: Zhao Zhili <zhilizhao@tencent.com>
Signed-off-by: Fei Wang <fei.w.wang@intel.com>

libavutil/hwcontext_vaapi.c

@@ -1319,8 +1319,17 @@ static int vaapi_map_to_drm_esh(AVHWFramesContext *hwfc, AVFrame *dst,
     surface_id = (VASurfaceID)(uintptr_t)src->data[3];
 
     export_flags = VA_EXPORT_SURFACE_SEPARATE_LAYERS;
-    if (flags & AV_HWFRAME_MAP_READ)
+    if (flags & AV_HWFRAME_MAP_READ) {
         export_flags |= VA_EXPORT_SURFACE_READ_ONLY;
+
+        vas = vaSyncSurface(hwctx->display, surface_id);
+        if (vas != VA_STATUS_SUCCESS) {
+            av_log(hwfc, AV_LOG_ERROR, "Failed to sync surface "
+                   "%#x: %d (%s).\n", surface_id, vas, vaErrorStr(vas));
+            return AVERROR(EIO);
+        }
+    }
+
     if (flags & AV_HWFRAME_MAP_WRITE)
         export_flags |= VA_EXPORT_SURFACE_WRITE_ONLY;