software_endstops: use *_MIN_POS and *_MAX_POS for arcs

If [XYZ]_HOME_POS and [XYZ]_MIN_POS aren't 0, these corrections are
wrong.  Use the same logic as in Marlin.pde:prepare_move: ie, clamp to
[XYZ]_{MIN,MAX}_POS.

While we're here, put this cut-and-paste code in a function
clamp_to_software_endstops.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

Marlin/Marlin.h

 
 void enquecommand(const char *cmd); //put an ascii command at the end of the current buffer.
 void prepare_arc_move(char isclockwise);
+void clamp_to_software_endstops(float target[3]);
 
 #ifdef FAST_PWM_FAN
 void setPwmFrequency(uint8_t pin, int val);

Marlin/Marlin.pde

    }
 }
 
-void prepare_move()
+void clamp_to_software_endstops(float target[3])
 {
   if (min_software_endstops) {
-    if (destination[X_AXIS] < X_MIN_POS) destination[X_AXIS] = X_MIN_POS;
-    if (destination[Y_AXIS] < Y_MIN_POS) destination[Y_AXIS] = Y_MIN_POS;
-    if (destination[Z_AXIS] < Z_MIN_POS) destination[Z_AXIS] = Z_MIN_POS;
+    if (target[X_AXIS] < X_MIN_POS) target[X_AXIS] = X_MIN_POS;
+    if (target[Y_AXIS] < Y_MIN_POS) target[Y_AXIS] = Y_MIN_POS;
+    if (target[Z_AXIS] < Z_MIN_POS) target[Z_AXIS] = Z_MIN_POS;
   }
 
   if (max_software_endstops) {
-    if (destination[X_AXIS] > X_MAX_POS) destination[X_AXIS] = X_MAX_POS;
-    if (destination[Y_AXIS] > Y_MAX_POS) destination[Y_AXIS] = Y_MAX_POS;
-    if (destination[Z_AXIS] > Z_MAX_POS) destination[Z_AXIS] = Z_MAX_POS;
+    if (target[X_AXIS] > X_MAX_POS) target[X_AXIS] = X_MAX_POS;
+    if (target[Y_AXIS] > Y_MAX_POS) target[Y_AXIS] = Y_MAX_POS;
+    if (target[Z_AXIS] > Z_MAX_POS) target[Z_AXIS] = Z_MAX_POS;
   }
+}
+
+void prepare_move()
+{
+  clamp_to_software_endstops(destination);
+
   previous_millis_cmd = millis();  
   plan_buffer_line(destination[X_AXIS], destination[Y_AXIS], destination[Z_AXIS], destination[E_AXIS], feedrate*feedmultiply/60/100.0, active_extruder);
   for(int8_t i=0; i < NUM_AXIS; i++) {

Marlin/motion_control.cpp

     arc_target[axis_linear] += linear_per_segment;
     arc_target[E_AXIS] += extruder_per_segment;
 
-    if (min_software_endstops) {
-      if (arc_target[X_AXIS] < X_HOME_POS) arc_target[X_AXIS] = X_HOME_POS;
-      if (arc_target[Y_AXIS] < Y_HOME_POS) arc_target[Y_AXIS] = Y_HOME_POS;
-      if (arc_target[Z_AXIS] < Z_HOME_POS) arc_target[Z_AXIS] = Z_HOME_POS;
-    }
-
-    if (max_software_endstops) {
-      if (arc_target[X_AXIS] > X_MAX_LENGTH) arc_target[X_AXIS] = X_MAX_LENGTH;
-      if (arc_target[Y_AXIS] > Y_MAX_LENGTH) arc_target[Y_AXIS] = Y_MAX_LENGTH;
-      if (arc_target[Z_AXIS] > Z_MAX_LENGTH) arc_target[Z_AXIS] = Z_MAX_LENGTH;
-    }
+    clamp_to_software_endstops(arc_target);
     plan_buffer_line(arc_target[X_AXIS], arc_target[Y_AXIS], arc_target[Z_AXIS], arc_target[E_AXIS], feed_rate, extruder);
     
   }