core:Window fix _density to default to 1. Check for changes in dens…

…ity before doing

sdl2/retina specific work

kivy/core/window/__init__.py

@@ -269,7 +269,6 @@ class WindowBase(EventDispatcher):
     __initialized = False
     _fake_fullscreen = False
     _density = 1
-    _win = None
 
     # private properties
     _size = ListProperty([0, 0])
@@ -955,10 +954,7 @@ def update_viewport(self):
         from math import radians
 
         w, h = self.system_size
-        # FIXME this is breaking the separation between the base and
-        # implementation. It should not happen, as now SDL2 implementation only
-        # got this.
-        if hasattr(self, "_win") and hasattr(self._win, '_get_gl_size'):
+        if self._density != 1:
             w, h = self.size
 
         smode = self.softinput_mode