Put all the thresholding code in the same place

This will make it easier to write tests validating the fast version.

Change-Id: I27cc372cc848c495d10a138b3c149d550dc0dad9
diff --git a/y2016/vision/target_sender.cc b/y2016/vision/target_sender.cc
index d9208ec..3e29085 100644
--- a/y2016/vision/target_sender.cc
+++ b/y2016/vision/target_sender.cc
@@ -98,8 +98,8 @@
     DecodeJpeg(data, &image_);
     auto fmt = image_.fmt();
 
-    RangeImage rimg =
-        DoThreshold(image_.get(), [](PixelRef &px) { return (px.g > 88); });
+    RangeImage rimg = ThresholdImageWithFunction(
+        image_.get(), [](PixelRef px) { return (px.g > 88); });
 
     // flip the right image as this camera is mount backward
     if (camera_index_ == 0) {