[WINED3D 4] Fix cmp instruction for GLSL

[Ivan Gyurdiev]

Note that both 3Dc and PhongLighting demos (which have the black-screen 
z-buffer problem) seem to be doing offscreen rendering. I know 
PhongLighting uses that to implement a shadow map. Now, pbuffers are 
currently disabled (that's why the shadow map is rendered on-screen, 
that causes the spinning objects in the lower left corner). Furthermore, 
maybe pbuffers are not working correctly - there's various comments 
about it in the disabled code. That's one area to investigate why those 
demos might not work properly.