Fix fdo#64751 – Header & footer blue tags empty

No idea FontMetric::GetHeight() is supposed to mean, probably the code
here meant GetLineHeight(). What it actually need is GetAscent(), so use
it instead (which is the same as GetLineHeight() - GetDescent()).

Change-Id: Ic467e56ed6b8d245ac7800f94e6d40030833328b
Reviewed-on: https://gerrit.libreoffice.org/3980Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>

sw/source/ui/docvw/HeaderFooterWin.cxx

@@ -291,7 +291,7 @@ void SwHeaderFooterWin::Paint( const Rectangle& )
     GetTextBoundRect( aTextRect, String( m_sLabel ) );
 
     FontMetric aFontMetric = GetFontMetric( GetFont() );
-    double nTextOffsetY = aFontMetric.GetHeight() - aFontMetric.GetDescent() + TEXT_PADDING;
+    double nTextOffsetY = aFontMetric.GetAscent() + TEXT_PADDING;
     Point aTextPos( TEXT_PADDING, nTextOffsetY );
 
     basegfx::B2DHomMatrix aTextMatrix( createScaleTranslateB2DHomMatrix(