Add XPT2046 calibration functions (#15140)

Marlin/src/feature/touch/xpt2046.cpp

   return (data[1] + data[2]) >> 1;
 }
 
+bool XPT2046::getTouchPoint(uint16_t &x, uint16_t &y) {
+  if (isTouched()) {
+    x = getInTouch(XPT2046_X);
+    y = getInTouch(XPT2046_Y);
+  }
+  return isTouched();
+}
+
 #endif // TOUCH_BUTTONS

Marlin/src/feature/touch/xpt2046.h

 public:
   static void init(void);
   static uint8_t read_buttons();
-private:
+  bool getTouchPoint(uint16_t &x, uint16_t &y);
   static bool isTouched();
+  inline void waitForRelease(void) { while (isTouched()) { /* nada */ } }
+  inline void waitForTouch(uint16_t &x, uint16_t &y) { while (!getTouchPoint(x, y)) { /* nada */ } }
+private:
   static uint16_t getInTouch(const XPTCoordinate coordinate);
 };