osx add the pixeldensity to screen info.

native/Avalonia.Native/inc/avalonia-native.h

@@ -52,6 +52,7 @@ struct AvnScreen
 {
     AvnRect Bounds;
     AvnRect WorkingArea;
+    float PixelDenisty;
     bool Primary;
 };
 

native/Avalonia.Native/src/OSX/Screens.mm

@@ -38,6 +38,8 @@ class Screens : public ComSingleObject<IAvnScreens, &IID_IAvnScreens>
             ret->WorkingArea.Height = [screen visibleFrame].size.height;
             ret->WorkingArea.Width = [screen visibleFrame].size.width;
             
+            ret->PixelDenisty = [screen backingScaleFactor];
+            
             ret->Primary = index == 0;
             
             return S_OK;