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/jevois/structures.h b/y2019/jevois/structures.h
index 1f10dc0..229af58 100644
--- a/y2019/jevois/structures.h
+++ b/y2019/jevois/structures.h
@@ -66,7 +66,7 @@
   // center of the camera's image plane to the center of the target.
   float distance;
 
-  // Height of the target in meters. Specifically, the distance from the floor
+  // Height of the target in meters. Specifically, the distance from the camera
   // to the center of the target.
   float height;