This works on cards that don't implement ARB_depth_clamp like r500
cards. Note that texturing is influenced by position.w, not position.z.

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.