horizon: fix resize freeze on Ubuntu

Avoids a potential resize loop as far as I can tell.  Ubuntu and MacOS deal with DPI differently in the matplotlib library and I currently can only test MacOS
Needs Testing!

Author: rbeall

MAVProxy/modules/lib/wxhorizon_ui.py

@@ -166,8 +166,10 @@ def checkReszie(self):
         if not self.resized:
             oldypx = self.ypx
             oldxpx = self.xpx
-            self.ypx = self.figure.get_size_inches()[1]*self.figure.dpi
-            self.xpx = self.figure.get_size_inches()[0]*self.figure.dpi
+            # use fixed DPI to match calcFontScaling()
+            dpi = 100.0
+            self.ypx = self.figure.get_size_inches()[1]*dpi
+            self.xpx = self.figure.get_size_inches()[0]*dpi
             if (oldypx != self.ypx) or (oldxpx != self.xpx):
                 self.resized = True
             else: