Add wxNativeFontInfo::GetPointSizeFromLogFontHeight() helper

No real changes, just refactor the code previously present in both
wxNativeFontInfo ctor and SetPixelSize() in a single function and reuse
it in both places.

include/wx/fontutil.h

@@ -118,7 +118,14 @@ public:
     // set the XFLD
     void SetXFontName(const wxString& xFontName);
 #elif defined(__WXMSW__)
-    wxNativeFontInfo(const LOGFONT& lf_);
+    wxNativeFontInfo(const LOGFONT& lf_)
+        : lf(lf_),
+          pointSize(GetPointSizeFromLogFontHeight(lf.lfHeight))
+    {
+    }
+
+    // MSW-specific: get point size from LOGFONT height using the default DPI.
+    static float GetPointSizeFromLogFontHeight(int height);
 
     // MSW-specific: get the height value in pixels using LOGFONT convention
     // (i.e. negative) corresponding to the given size in points and DPI.