wined3d: Fix compile for older OpenGL headers.

[H. Verbeet]

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.