Avoid unnecessary calls in wxGDIPlusContext::GetTextExtent()

Don't always call several GDI+ functions when we often don't need their
results: only retrieve the cell descent, ascent and the line spacing if
we are going to really use any of these values.

src/msw/graphics.cpp

@@ -2111,9 +2111,12 @@ void wxGDIPlusContext::GetTextExtent( const wxString &str, wxDouble *width, wxDo
     wxCHECK_RET( !m_font.IsNull(), wxT("wxGDIPlusContext::GetTextExtent - no valid font set") );
 
     wxWCharBuffer s = str.wc_str( *wxConvUI );
-    FontFamily ffamily ;
     Font* f = ((wxGDIPlusFontData*)m_font.GetRefData())->GetGDIPlusFont();
 
+    // Get the font metrics if we actually need them.
+    if ( descent || externalLeading || (height && str.empty()) )
+    {
+        FontFamily ffamily ;
         f->GetFamily(&ffamily) ;
 
         REAL factorY = m_fontScaleRatio;
@@ -2127,17 +2130,21 @@ void wxGDIPlusContext::GetTextExtent( const wxString &str, wxDouble *width, wxDo
         REAL rAscent = ffamily.GetCellAscent(style) * size / emHeight;
         REAL rHeight = ffamily.GetLineSpacing(style) * size / emHeight;
 
-    if ( height )
+        if ( height && str.empty() )
             *height = rHeight * factorY;
         if ( descent )
             *descent = rDescent * factorY;
         if ( externalLeading )
             *externalLeading = (rHeight - rAscent - rDescent) * factorY;
+    }
+
     // measuring empty strings is not guaranteed, so do it by hand
     if ( str.IsEmpty())
     {
         if ( width )
             *width = 0 ;
+
+        // Height already assigned above if necessary.
     }
     else
     {