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/control_loops/drivetrain/camera.h b/y2019/control_loops/drivetrain/camera.h
index 8015772..04e4157 100644
--- a/y2019/control_loops/drivetrain/camera.h
+++ b/y2019/control_loops/drivetrain/camera.h
@@ -108,7 +108,7 @@
       Scalar heading;   // radians
       // The distance from the camera to the target.
       Scalar distance;  // meters
-      // Height of the target from the floor.
+      // Height of the target from the camera.
       Scalar height;    // meters
       // The angle of the target relative to the frame of the camera.
       Scalar skew;      // radians
@@ -292,7 +292,7 @@
   view.reading.heading = heading;
   view.reading.distance = distance;
   view.reading.skew = skew;
-  view.reading.height = target.pose().abs_pos().z();
+  view.reading.height = relative_pose.rel_pos().z();
   view.target = ⌖
   view.camera_pose = camera_abs_pose;