Rename all qgl* functions top their OpenGL counterparts

This removes the need to define the old qgl function names to the
official OpenGL names. The OpenGL functions are now called directly
without any abstraction.
Remove support for old SGI Opengl extensions

Those extensions have become part of ARB about 15 years ago and most if
not all video cards still in use should support the ARB versions. I
believe that at least parts of this code were disfunctional.
Call GLimp_GetProcAddress directly