Gtk.DrawTextLayout: correct positioning if layout.Height>0

CairoContextBackendHandler.DrawTextLayout 
positions Text on different locations depending on TextLayout.Height:

![drawtextgtkerror](https://f.cloud.github.com/assets/1479980/1984208/b1aa4772-8426-11e3-9c9c-72d89820d64a.PNG)

see DrawingText-Example:
draw with textlayout.Height = -1
<code>
ctx.SetColor (Colors.Red);
ctx.DrawTextLayout (tl0, lotPos);
</code>
 draw with textlayout.Height = lotSize.Height
<code>
ctx.SetColor (Colors.Black);
tl0.Height = lotSize.Height;
ctx.DrawTextLayout (tl0, lotPos.X, lotPos.Y);
</code>