lavfi/overlay_vaapi: set input region for overlay video to NULL

Otherwise main and overlay videos share the same input region. Note NULL
pointer imples the whole overlay video will be processed.

Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>

libavfilter/vf_overlay_vaapi.c

@@ -237,6 +237,7 @@ static int overlay_vaapi_blend(FFFrameSync *fs)
         params[1].blend_state = &blend_state;
 
         params[1].surface       = (VASurfaceID)(uintptr_t)input_overlay->data[3];
+        params[1].surface_region = NULL;
         params[1].output_region = &overlay_region;
     }