a new (slightly inelegant) stab at eliminating the slow tail problem

planner.h

@@ -45,6 +45,7 @@ typedef struct {
   
   // Settings for the trapezoid generator
   uint32_t initial_rate;              // The jerk-adjusted step rate at start of block  
+  uint32_t final_rate;                // The minimal rate at exit
   int32_t rate_delta;                 // The steps/minute to add or subtract when changing speed (must be positive)
   uint32_t accelerate_until;          // The index of the step event on which to stop acceleration
   uint32_t decelerate_after;          // The index of the step event on which to start decelerating