Introduce M221 aka flow-control.

Now it is possible to control the extruders flow.

  M221 S100 = 100% of the extruders steps
  M221 S90  = 90% of the extruders steps

M221 is also used in other firmwares for this. Also a lot of
hosts, like Octoprint and Pronterface using this M-Code for
this behaviour.

dda.c

@@ -110,6 +110,8 @@ void dda_init(void) {
 	// set up default feedrate
 	if (startpoint.F == 0)
 		startpoint.F = next_target.target.F = SEARCH_FEEDRATE_Z;
+  if (startpoint.e_multiplier == 0)
+    startpoint.e_multiplier = next_target.target.e_multiplier = 100;
 }
 
 /*! Distribute a new startpoint to DDA's internal structures without any movement.
@@ -216,6 +218,11 @@ void dda_create(DDA *dda, TARGET *target) {
   // type, but are subject to other special handling.
   steps[E] = um_to_steps(target->axis[E], E);
 
+  // Apply extrusion multiplier.
+  steps[E] *= target->e_multiplier;
+  steps[E] += 50;
+  steps[E] /= 100;
+
   if ( ! target->e_relative) {
     int32_t delta_steps;
 

dda.h

@@ -41,6 +41,7 @@ typedef struct {
   axes_int32_t axis;
   uint32_t  F;
 
+  uint8_t   e_multiplier;
   uint8_t   e_relative        :1; ///< bool: e axis relative? Overrides all_relative
 } TARGET;
 

gcode_process.c

@@ -793,6 +793,13 @@ void process_gcode_command() {
 				#endif
 				break;
 
+      case 221:
+        //? --- M221: Control the extruders flow ---
+        if ( ! next_target.seen_S)
+          break;
+        next_target.target.e_multiplier = next_target.S;
+        break;
+
       #ifdef DEBUG
 			case 240:
 				//? --- M240: echo off ---