Allow cancelling un-executed splines

Previously we couldn't cancel a spline without starting execution of it.

Also, allow specifying driving backwards on a per-spline basis.

Change-Id: I7a1fc54a49bfdcfe8c8614bde18d2976ba3a7868
diff --git a/frc971/control_loops/control_loops.q b/frc971/control_loops/control_loops.q
index a8e6b8c..2359bcd 100644
--- a/frc971/control_loops/control_loops.q
+++ b/frc971/control_loops/control_loops.q
@@ -216,5 +216,8 @@
   float[36] spline_x;
   float[36] spline_y;
 
+  // Whether to follow the spline driving forwards or backwards.
+  bool drive_spline_backwards;
+
   Constraint[6] constraints;
 };