[WINED3D 4] Fix cmp instruction for GLSL

[Ivan Gyurdiev]

Ivan Gyurdiev wrote:
> 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.
Also note that both 3Dc and PhongLighting are using unsupported 
floating-point formats (R16F, and R32F) for the offscreen shadow map. 
So, it isn't surprising at all that shadows aren't working correctly 
even with pbuffers enabled (which I tried).