Optionally don't invert pressure readings.

This is required e.g. on ILI9341. Default to the old behavior to not
break any existing projects.

Author: rtwfroody

adafruit_touchscreen.py

@@ -104,7 +104,8 @@ def __init__(
         samples: int = 4,
         z_threshold: int = 10000,
         calibration: Optional[Tuple[Tuple[int, int], Tuple[int, int]]] = None,
-        size: Optional[Tuple[int, int]] = None
+        size: Optional[Tuple[int, int]] = None,
+        invert_pressure: bool = True
     ) -> None:
 
         self._xm_pin = x1_pin
@@ -119,6 +120,7 @@ def __init__(
         self._calib = calibration
         self._size = size
         self._zthresh = z_threshold
+        self.invert_pressure = invert_pressure
 
     @property
     def touch_point(
@@ -136,7 +138,10 @@ def touch_point(
                 with AnalogIn(self._yp_pin) as y_p:
                     z_2 = y_p.value
         # print(z_1, z_2)
-        z = 65535 - (z_2 - z_1)
+        if self.invert_pressure:
+            z = 65535 - (z_2 - z_1)
+        else:
+            z = z_2 + z_1
         if z > self._zthresh:
             with DigitalInOut(self._yp_pin) as y_p:
                 with DigitalInOut(self._ym_pin) as y_m: