Text vertical alignment

These patches provides some primitive initial support for vertical alignment of text in CairoSVG. The first patch ea37bd2 provides some limited support for the SVG 1.1 "alignment-baseline" attribute (for the "text" tag only). The second patch 80e666d goes beyond the SVG specification to provide vertical alignment of the specific text rather than its font baseline.

Even though these patches don't completely solve the vertical text alignment, it would be great if you could take them. Right now, I know of no open source SVG renderer that supports vertical text alignment at all, and it's a pretty huge functionality hole for applications that are generating SVG without access to the underlying font metrics.