Speed up QUICK_HOME feedrate for diagonal move

Speed up the diagonal move while still keeping each individual axis at
or below its homing feedrate.

Marlin/Marlin_main.cpp

         destination[X_AXIS] = 1.5 * max_length(X_AXIS) * x_axis_home_dir;destination[Y_AXIS] = 1.5 * max_length(Y_AXIS) * home_dir(Y_AXIS);
         feedrate = homing_feedrate[X_AXIS];
         if(homing_feedrate[Y_AXIS]<feedrate)
-          feedrate =homing_feedrate[Y_AXIS];
+          feedrate = homing_feedrate[Y_AXIS];
+        if (max_length(X_AXIS) > max_length(Y_AXIS)) {
+          feedrate *= sqrt(pow(max_length(Y_AXIS) / max_length(X_AXIS), 2) + 1);
+        } else {
+          feedrate *= sqrt(pow(max_length(X_AXIS) / max_length(Y_AXIS), 2) + 1);
+        }
         plan_buffer_line(destination[X_AXIS], destination[Y_AXIS], destination[Z_AXIS], destination[E_AXIS], feedrate/60, active_extruder);
         st_synchronize();