Text engine for OpenGL renderer

[Michael Goffioul]

Hi,

For those interested, I just commited a new Freetype/Fontconfig based text
engine for the OpenGL backend. At the moment, it's very basic. It's mainly
a skeleton that contains enough to render tick labels, axes labels, titles and
simple text objects. It will be enhanced later on.

Note that if fontconfig is not available, the engine uses a hardcoded
font, which
defaults to Verdana under Win32 platform. As I didn't what font to use under
UNIX platform, I left that part empty. This means that if you're running UNIX
without fontconfig (development package), text rendering will probably not work.

Enjoy,
Michael.