Initialize wxFontRefData::m_sizeUsingPixels in wxMSW

Don't leave the variable uninitialized, even though it's not totally obvious
which value should it have for wxFontRefData objects created from native
fonts, using "true" seems more appropriate and is definitely better than not
initializing the field at all.

(this is a backport of 2f1e8c5402 from master)

src/msw/font.cpp

@@ -386,7 +386,8 @@ void wxFontRefData::Init(const wxNativeFontInfo& info, WXHFONT hFont)
     m_hFont = (HFONT)hFont;
     m_nativeFontInfo = info;
 
-    // TODO: m_sizeUsingPixels?
+    // size of native fonts is expressed in pixels
+    m_sizeUsingPixels = true;
 }
 
 wxFontRefData::~wxFontRefData()