[Scenic] Fix Renderer::Visitor Crash

Supply a BatchGpuUploader to clipper visitors and relax DCHECK for
visited Materials.
The clipper VisitorContext is marked kNoMaterial, but its visitor can
still visit a Material. The upload is a no-op in this case, but
removed the DCHECK so it doesn't crash on debug builds

SCN-1245#done

Tested:
- ran in debug mode

Change-Id: I93efbf3077e0d377018e95acbcfdf8e4c7d033d8
1 file changed