wined3d: Fix compile for older OpenGL headers.

[Vitaly Budovski]

H. Verbeet wrote:
> Although #ifdef GL_VERSION_2_0 will make compilation work with older
> OpenGl headers, it's not the way to fix this. Basically, wined3d
> should check for functionallity during runtime with GL_SUPPORT and
> call extensions with GL_EXTCALL. The issue has come up a few times
> before with patches from the same author, and I think he should be
> aware of the issue by now.
>
>   
That takes care of functions as part of an OpenGL extension.
What about core functions of a newer GL version? Would all of
those need to be defined in the wined3d_gl.h file?