Revert "opengl32: Return a NULL pointer for functions requiring unsupported or disabled extensions.".

[Henri Verbeet]

This is wrong. I we really want to add hacks for specific combinations
of broken drivers and applications, we should be adding exceptions for
specific extensions / functions. In the specific example of bug 38480,
it should be easy enough to implement the required functionality on
top of the core functionality in OpenGL 1.2 if the extension is not
available.