CairoContextbackendHandler: place first line on baseline, not on descender

this is a follow up of https://github.com/mono/xwt/pull/277

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)

reason:  pango_layout_set_text sets text origin to top/left, pango_cairo_show_layout_line to the BASELINE of the text  (see: http://cairographics.org/FAQ/)
h+extl.Height means setting to the descender line

![500px-typography_line_terms svg](https://f.cloud.github.com/assets/1479980/2013103/247c66dc-8791-11e3-88a3-3a59258d07f6.png)

so the position of the first line has to be the baseline
the baseline can calculated with Context.FontExtents.Ascent and Descent  (approx. 75% of the font height in most fonts).