dda_queue.c/.h: introduce mb_tail_dda.

For now, this costs 2 bytes RAM, 8 bytes binary size and slows
down the slowest step by 4 clock cycles. We expect opportunities
for improvements elsewhere, of course.

  ATmega sizes               '168   '328(P)   '644(P)     '1280
  Program:  19434 bytes      136%       64%       31%       16%
     Data:   2179 bytes      213%      107%       54%       27%
   EEPROM:     32 bytes        4%        2%        2%        1%

  short-moves.gcode statistics:
  LED on occurences: 888.
  LED on time minimum: 230 clock cycles.
  LED on time maximum: 407 clock cycles.
  LED on time average: 263.008 clock cycles.

  smooth-curves.gcode statistics:
  LED on occurences: 23648.
  LED on time minimum: 251 clock cycles.
  LED on time maximum: 450 clock cycles.
  LED on time average: 286.212 clock cycles.

  triangle-odd.gcode statistics:
  LED on occurences: 1636.
  LED on time minimum: 251 clock cycles.
  LED on time maximum: 407 clock cycles.
  LED on time average: 276.568 clock cycles.

dda_queue.c

@@ -36,6 +36,12 @@ uint8_t	mb_tail = 0;
 /// The size does not need to be a power of 2 anymore!
 DDA BSS movebuffer[MOVEBUFFER_SIZE];
 
+/**
+  Pointer to the currently ongoing movement, or NULL, if there's no movement
+  ongoing. Actually a cache of movebuffer[mb_tail].
+*/
+DDA *mb_tail_dda;
+
 /// Find the next DDA index after 'x', where 0 <= x < MOVEBUFFER_SIZE
 #define MB_NEXT(x) ((x) < MOVEBUFFER_SIZE - 1 ? (x) + 1 : 0)
 
@@ -89,7 +95,11 @@ void queue_step() {
   if ( ! movebuffer[mb_tail].live) {
     if (mb_tail != mb_head) {
       mb_tail = MB_NEXT(mb_tail);
-      dda_start(&movebuffer[mb_tail]);
+      mb_tail_dda = &(movebuffer[mb_tail]);
+      dda_start(mb_tail_dda);
+    }
+    else {
+      mb_tail_dda = NULL;
     }
   }
 }
@@ -140,7 +150,8 @@ void enqueue_home(TARGET *t, uint8_t endstop_check, uint8_t endstop_stop_cond) {
       */
       timer_reset();
       mb_tail = mb_head;  // Valid ONLY if the queue was empty before!
-      dda_start(&movebuffer[mb_tail]);
+      mb_tail_dda = new_movebuffer; // Dito!
+      dda_start(mb_tail_dda);
       // Compensate for the cli() in timer_set().
       sei();
     }
@@ -162,7 +173,7 @@ void queue_flush() {
   // if the timer were running, this would require
   // wrapping in ATOMIC_START ... ATOMIC_END.
   mb_tail = mb_head;
-  movebuffer[mb_head].live = 0;
+  mb_tail_dda = NULL;
 }
 
 /// wait for queue to empty

dda_queue.h

@@ -13,6 +13,8 @@
 // this is the ringbuffer that holds the current and pending moves.
 extern uint8_t mb_tail;
 extern DDA movebuffer[MOVEBUFFER_SIZE];
+extern DDA *mb_tail_dda;
+
 
 /*
   methods