BF: with a retina display and Window.units='pix' mouse.getPos needs /2

The (actual) coords returned by the retina display are twice the
(virtual) coords returned by the window. So mouse.getPos needs to convert
these back to virtual coords as well (by dividing by 2)

psychopy/event.py

@@ -643,6 +643,7 @@ def getPos(self):
             else:
                 lastPosPix = lastPosPix - numpy.array(self.win.size) / 2
         self.lastPos = self._pix2windowUnits(lastPosPix)
+
         return copy.copy(self.lastPos)
 
     def mouseMoved(self, distance=None, reset=False):
@@ -849,6 +850,8 @@ def isPressedIn(self, shape, buttons=(0, 1, 2)):
 
     def _pix2windowUnits(self, pos):
         if self.win.units == 'pix':
+            if self.win.useRetina:
+                pos /= 2.0
             return pos
         elif self.win.units == 'norm':
             return pos * 2.0 / self.win.size