Fix broken test

Most people never run this test because its dependencies aren't
well-specified, so once you sort that out you get to fix the bugs that
have existed in it for a while...

Change-Id: Ic72d5d3611637b70ddb2d23b53cdd35cf2b2cec3
diff --git a/aos/vision/tools/jpeg_vision_test.cc b/aos/vision/tools/jpeg_vision_test.cc
index 806fd80..5267041 100644
--- a/aos/vision/tools/jpeg_vision_test.cc
+++ b/aos/vision/tools/jpeg_vision_test.cc
@@ -74,7 +74,7 @@
     prev_data_ = data.to_string();
 
     // Threshold the image with the given lambda.
-    RangeImage rimg = ThresholdImageWithFunction(img_ptr, [](PixelRef &px) {
+    RangeImage rimg = ThresholdImageWithFunction(img_ptr, [](PixelRef px) {
       if (px.g > 88) {
         uint8_t min = std::min(px.b, px.r);
         uint8_t max = std::max(px.b, px.r);