XML-metadata-based GLSL shader framework

Geometry shader

I have slightly modified the core source in order to offer support for geometry shader through the "GL_EXT_geometry_shader4" extension. This update's implemention is based on Xie Yongming's Geometry Shader Tutorials, yet the implementation has not been thouroughly tested.

There are no new release packages, only the SVN version updated, though.

I will need to test this functionality, especially the shader switching part for geo-shaders before I can create a new release.