Tweaks to motion.h reachable tests

Marlin/src/module/motion.h

   #include "../module/scara.h"
 #endif
 
+// Error margin to work around float imprecision
+constexpr float slop = 0.0001;
+
 extern bool relative_mode;
 
 extern float current_position[XYZE],  // High-level current tool position
 
 #else // CARTESIAN
 
-   // Return true if the given position is within the machine bounds.
+  // Return true if the given position is within the machine bounds.
   inline bool position_is_reachable(const float &rx, const float &ry) {
+    if (!WITHIN(ry, Y_MIN_POS - slop, Y_MAX_POS + slop)) return false;
     #if ENABLED(DUAL_X_CARRIAGE)
-      if (active_extruder == 0) {
-        // Add 0.001 margin to deal with float imprecision
-        return WITHIN(rx, X1_MIN_POS - 0.001f, X1_MAX_POS + 0.001f)
-            && WITHIN(ry,  Y_MIN_POS - 0.001f,  Y_MAX_POS + 0.001f);
-      } else {
-       // Add 0.001 margin to deal with float imprecision
-       return WITHIN(rx, X2_MIN_POS - 0.001f, X2_MAX_POS + 0.001f)
-           && WITHIN(ry,  Y_MIN_POS - 0.001f,  Y_MAX_POS + 0.001f);
-      }
+      if (active_extruder)
+        return WITHIN(rx, X2_MIN_POS - slop, X2_MAX_POS + slop);
+      else
+        return WITHIN(rx, X1_MIN_POS - slop, X1_MAX_POS + slop);
     #else
-      // Add 0.001 margin to deal with float imprecision
-      return WITHIN(rx, X_MIN_POS - 0.001f, X_MAX_POS + 0.001f)
-          && WITHIN(ry, Y_MIN_POS - 0.001f, Y_MAX_POS + 0.001f);
+      return WITHIN(rx, X_MIN_POS - slop, X_MAX_POS + slop);
     #endif
   }
 
      */
     inline bool position_is_reachable_by_probe(const float &rx, const float &ry) {
       return position_is_reachable(rx - (X_PROBE_OFFSET_FROM_EXTRUDER), ry - (Y_PROBE_OFFSET_FROM_EXTRUDER))
-          && WITHIN(rx, MIN_PROBE_X - 0.001f, MAX_PROBE_X + 0.001f)
-          && WITHIN(ry, MIN_PROBE_Y - 0.001f, MAX_PROBE_Y + 0.001f);
+          && WITHIN(rx, MIN_PROBE_X - slop, MAX_PROBE_X + slop)
+          && WITHIN(ry, MIN_PROBE_Y - slop, MAX_PROBE_Y + slop);
     }
   #endif