Add OSXMakeDPIFromScaleFactor() helper

No real changes, just avoid repeating wxRound(scaleFactor*72) in several
places and do it in this single function instead.

include/wx/osx/window.h

@@ -294,6 +294,9 @@ public:
     // internal response to size events
     virtual void MacOnInternalSize() {}
 
+    // Return the DPI corresponding to the given scale factor.
+    static wxSize       OSXMakeDPIFromScaleFactor(double scaleFactor);
+
 protected:
     // For controls like radio buttons which are genuinely composite
     wxList              m_subControls;

src/osx/cocoa/nonownedwnd.mm

@@ -629,8 +629,8 @@ extern int wxOSXGetIdFromSelector(SEL action );
                                          doubleValue];
         if (newBackingScaleFactor != oldBackingScaleFactor)
         {
-            wxSize oldDPI((int)(oldBackingScaleFactor*72.0),(int)(oldBackingScaleFactor*72.0));
+            const wxSize oldDPI = wxWindow::OSXMakeDPIFromScaleFactor(oldBackingScaleFactor);
-            wxSize newDPI((int)(newBackingScaleFactor*72.0),(int)(newBackingScaleFactor*72.0));
+            const wxSize newDPI = wxWindow::OSXMakeDPIFromScaleFactor(newBackingScaleFactor);
 
             wxDPIChangedEvent event(oldDPI, newDPI);
             event.SetEventObject(wxpeer);

src/osx/window_osx.cpp

@@ -2620,6 +2620,14 @@ bool wxWindowMac::OSXHandleKeyEvent( wxKeyEvent& event )
     return handled ;
 }
 
+/* static */
+wxSize wxWindowMac::OSXMakeDPIFromScaleFactor(double scaleFactor)
+{
+    const int dpi = wxRound(scaleFactor*72.0);
+
+    return wxSize(dpi, dpi);
+}
+
 wxSize wxWindowMac::GetDPI() const
 {
     double scaleFactor;
@@ -2628,7 +2636,7 @@ wxSize wxWindowMac::GetDPI() const
     else
         scaleFactor = wxOSXGetMainScreenContentScaleFactor();
 
-    return wxSize(wxRound(scaleFactor*72.0),wxRound(scaleFactor*72.0));
+    return OSXMakeDPIFromScaleFactor(scaleFactor);
 }
 
 //