The NVfp2 control flow uses the D3D syntax, and doesn't need the inversed
branch condition we're using in the vertex shader.

I don't like that I have to do this because the posFixup is in all
vertex programs, so its at the same position and could be loaded
globally.  Unfortunately, there are only 256 env parameters usually,
which makes it impossible for any shader to use c256, even if it does
not use indirect addressing, and so we can't claim 256 constant
support.

The best is to put this together with the rest of the shader_arb_priv initialization.

I find it helpful for debugging to have this controlled at a central place,
without having to disable the entire GL extension or manually find all the
places where GL_SUPPORT(NV_VERTEX_PROGRAM2_OPTION) controls clipplane use. It
is useful for debugging the emulation code on NV cards and for debugging mac
driver issues.