wined3d: Make pixelshaders disable fog properly.

wined3d: Make pixelshaders disable fog properly.

This is a first step towards cleaning up the fog mess. The fog
parameter is added to the pixelshader compile args structure. That way
multiple pshaders are compiled for different fog settings, and the
pixel shader can remove the fog line if fog is not enabled. That way
we don't need special fog start and end settings, and this allows us
to implement EXP and EXP2 fog in the future too.