wined3d: Add a function to control use of NV_vp2 clipplanes.

wined3d: Add a function to control use of NV_vp2 clipplanes.

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.