OpenGL improvements

OpenGL extensions can be querried at runtime using the glGetString(GL_EXTENSIONS ) call. Both rectangular textures and yuv textures may beavailable at non-apple platform (or even not available on some macs).

More fancy stuff can be done with GL, like subpicture rendering.

With recent hardware (and opengl shading language) stuff like dct, motioncompensation or postprocessing should be possible. Also see #29 (closed)