The fog settings do not depend on wether the shader writes to oFog or not,
instead they depend on the FOGVERTEXMODE and FOGTABLEMODE settings, and if a
vertex shader is bound at all.
It works the same way as with the fixed function, and having a vertex shader
is the same as using pretransformed vertices, just that the fog coord comes
from the shader instead of the specular color:
FOGTABLEMODE != NONE: The Z coord is used, oFog is ignored
FOGTABLEMODE == NONE, with VS: oFog is used
FOGTABLEMODE == NONE, no VS, XYZ: Z is used
FOGTABLEMODE == NONE, no VS, XYZRHW: diffuse color is used
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