Add 7971 front/back camera constants and fix height reference

The height reference for the targets is actually supposed to be from the
camera, not the floor.

Change-Id: Iee5a23230b4b1fbc6f0f1ebaf5344135cc78b14a
diff --git a/y2019/vision/constants.cc b/y2019/vision/constants.cc
index 6238cb1..fa7c130 100644
--- a/y2019/vision/constants.cc
+++ b/y2019/vision/constants.cc
@@ -59,6 +59,8 @@
      55,
     }};
 
+// Note: x/y should be negated and heading should be offset by 180 deg to
+// account for this being calibrated on the rear of the robot.
 CameraCalibration camera_19 = {
     {
      -0.341036 / 180.0 * M_PI, 324.626, 1.2545 / 180.0 * M_PI,