The Z disable part of the test is technically not necessary because the
fog coordinate is written before applying the projection matrix. I've
nevertheless included it because this is not immediately obvious and fog
is the only place where the vertex Z matters if depth test and depth
clipping are disabled.

Buffers don't have a format as such, and are created with either
WINED3DFMT_UNKNOWN or WINED3DFMT_VERTEXDATA.

This function is only used in the non-default "backbuffer" ORM.

GL_CLAMP was removed from core profile and probably GL_CLAMP_TO_EDGE was
intended to be used here anyway.