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/y2019/vision/global_calibration.cc b/y2019/vision/global_calibration.cc
index 3d1e91f..304f4cb 100644
--- a/y2019/vision/global_calibration.cc
+++ b/y2019/vision/global_calibration.cc
@@ -153,7 +153,7 @@
 
     const ::aos::vision::ImageFormat fmt{640, 480};
     ::aos::vision::BlobList imgs =
-        ::aos::vision::FindBlobs(aos::vision::DoThresholdYUYV(
+        ::aos::vision::FindBlobs(aos::vision::SlowYuyvYThreshold(
             fmt, frame.data.data(), TargetFinder::GetThresholdValue()));
     target_finder.PreFilter(&imgs);