[coretext] Fix line spacing

It should have been nExtDescent + pMetric->mnDescent, but then the whole
thing does not make any sense; why add the descent to the leading and
then calculate the leading by subtracting the descent! (well, the ATSUI
code was doing this but it makes no sense either).

Just use CTFontGetLeading() directly.

Change-Id: Ia54648f6c02c11359865f4aa6476adf40b27f906

vcl/coretext/salgdi.cxx

@@ -151,9 +151,7 @@ void QuartzSalGraphics::GetFontMetric( ImplFontMetricData* pMetric, int nFallbac
 
     pMetric->mnAscent = static_cast<long>( CTFontGetAscent(font) * mfFakeDPIScale + 0.5);
     pMetric->mnDescent = static_cast<long>(CTFontGetDescent(font) * mfFakeDPIScale + 0.5);
-    const long nExtDescent  = static_cast<long>((CTFontGetLeading(font) + CTFontGetDescent(font)) *
-                                                mfFakeDPIScale + 0.5);
-    pMetric->mnExtLeading   = nExtDescent + pMetric->mnDescent;
+    pMetric->mnExtLeading = static_cast<long>(CTFontGetLeading(font) * mfFakeDPIScale + 0.5);
     pMetric->mnIntLeading   = 0;
     pMetric->mnWidth = m_style->GetFontStretchedSize();