Fix backwards spline feedback

We weren't handling the logic for feedback on backwards splines
correctly.

TODO: Clean up some of the backwards logic to make it possible to
follow.

Change-Id: I0c18b8c245010151356935dfcbf8babdb98d0846
diff --git a/frc971/control_loops/drivetrain/trajectory.h b/frc971/control_loops/drivetrain/trajectory.h
index 00b37f6..60695de 100644
--- a/frc971/control_loops/drivetrain/trajectory.h
+++ b/frc971/control_loops/drivetrain/trajectory.h
@@ -206,7 +206,8 @@
   // point (i.e., distance should be roughly equal to the actual distance along
   // the path).
   Eigen::Matrix<double, 5, 1> StateToPathRelativeState(
-      double distance, const Eigen::Matrix<double, 5, 1> &state) const;
+      double distance, const Eigen::Matrix<double, 5, 1> &state,
+      bool drive_backwards) const;
 
   // Retrieves the gain matrix K for a given distance along the path.
   Eigen::Matrix<double, 2, 5> GainForDistance(double distance) const;