Dramatically speed up measuring text extent in wxMSW

Skip correction for the under/overhang for non-italic fonts: it seems to
be pretty small for them and avoiding the calls to ::GetCharABCWidths()
makes GetTextExtent() 7-8 times faster, which seems to be a worthwhile
compensation.

If we decide to restore these calls in the future, we will need to
implement some kind of cache for them, as they're just too slow
otherwise.

src/msw/textmeasure.cpp

@@ -108,7 +108,12 @@ void wxTextMeasure::DoGetTextExtent(const wxString& string,
     // the result computed by GetTextExtentPoint32() may be too small as it
     // accounts for under/overhang of the first/last character while we want
     // just the bounding rect for this string so adjust the width as needed
-    if ( len > 0 )
+    // when using italic fonts as the difference is really noticeable for them
+    // (it may still exist, but seems to be at most 1px for the other fonts,
+    // and calling GetCharABCWidths() is pretty slow and much slower than
+    // calling GetTextExtentPoint32() itself, so avoid its overhead unless it's
+    // really, really necessary).
+    if ( GetFont().GetStyle() != wxFONTSTYLE_NORMAL && len > 0 )
     {
         ABC widthABC;
         const wxChar chFirst = *string.begin();