Fix CoreXY speed calculation

For cartesian bots, the X_AXIS is the real X movement and same for
Y_AXIS.
But for corexy bots, that is not true. The "X_AXIS" and "Y_AXIS" motors
(that should be named to A_AXIS
and B_AXIS) cannot be used for X and Y length, because A=X+Y and B=X-Y.
So we need to create other 2 "AXIS", named X_HEAD and Y_HEAD, meaning
the real displacement of the Head.
Having the real displacement of the head, we can calculate the total
movement length and apply the desired speed.

Marlin/Marlin.h

@@ -171,7 +171,7 @@ void manage_inactivity(bool ignore_stepper_queue=false);
 #endif
 
 
-enum AxisEnum {X_AXIS=0, Y_AXIS=1, Z_AXIS=2, E_AXIS=3};
+enum AxisEnum {X_AXIS=0, Y_AXIS=1, Z_AXIS=2, E_AXIS=3, X_HEAD=4, Y_HEAD=5};
 
 
 void FlushSerialRequestResend();

Marlin/planner.cpp

@@ -715,11 +715,21 @@ block->steps_y = labs((target[X_AXIS]-position[X_AXIS]) - (target[Y_AXIS]-positi
     if(feed_rate<minimumfeedrate) feed_rate=minimumfeedrate;
   } 
 
-  float delta_mm[4];
+/* This part of the code calculates the total length of the movement. 
+For cartesian bots, the X_AXIS is the real X movement and same for Y_AXIS.
+But for corexy bots, that is not true. The "X_AXIS" and "Y_AXIS" motors (that should be named to A_AXIS
+and B_AXIS) cannot be used for X and Y length, because A=X+Y and B=X-Y.
+So we need to create other 2 "AXIS", named X_HEAD and Y_HEAD, meaning the real displacement of the Head. 
+Having the real displacement of the head, we can calculate the total movement length and apply the desired speed.
+*/ 
   #ifndef COREXY
+    float delta_mm[4];
     delta_mm[X_AXIS] = (target[X_AXIS]-position[X_AXIS])/axis_steps_per_unit[X_AXIS];
     delta_mm[Y_AXIS] = (target[Y_AXIS]-position[Y_AXIS])/axis_steps_per_unit[Y_AXIS];
   #else
+    float delta_mm[6];
+    delta_mm[X_HEAD] = (target[X_AXIS]-position[X_AXIS])/axis_steps_per_unit[X_AXIS];
+    delta_mm[Y_HEAD] = (target[Y_AXIS]-position[Y_AXIS])/axis_steps_per_unit[Y_AXIS];
     delta_mm[X_AXIS] = ((target[X_AXIS]-position[X_AXIS]) + (target[Y_AXIS]-position[Y_AXIS]))/axis_steps_per_unit[X_AXIS];
     delta_mm[Y_AXIS] = ((target[X_AXIS]-position[X_AXIS]) - (target[Y_AXIS]-position[Y_AXIS]))/axis_steps_per_unit[Y_AXIS];
   #endif
@@ -731,7 +741,11 @@ block->steps_y = labs((target[X_AXIS]-position[X_AXIS]) - (target[Y_AXIS]-positi
   } 
   else
   {
+    #ifndef COREXY
       block->millimeters = sqrt(square(delta_mm[X_AXIS]) + square(delta_mm[Y_AXIS]) + square(delta_mm[Z_AXIS]));
+	#else
+	  block->millimeters = sqrt(square(delta_mm[X_HEAD]) + square(delta_mm[Y_HEAD]) + square(delta_mm[Z_AXIS]));
+    #endif	
   }
   float inverse_millimeters = 1.0/block->millimeters;  // Inverse millimeters to remove multiple divides