st/nine: Use perspective correction for ps depth fog

st/nine: Use perspective correction for ps depth fog

Emulate perspective interpolation of depth for programmable ps fog

ff ps fog uses position z, or 1/w depending on the ff
projection matrix set. This is according to public documents
found describing the algorithm and tests we made.

In the case of programmable ps, we used position's z,
which was sufficient to pass wine tests (which test shaders
don't set w).

Issue https://github.com/iXit/Mesa-3D/issues/315 showed
that this calculation was wrong.
Using perspective interpolation on z, that is using z * 1/w
seems to satisfy both this application and wine tests.

Fixes: https://github.com/iXit/Mesa-3D/issues/315
Signed-off-by: Axel Davy <davyaxel0@gmail.com>