wined3d: Introduce a separate function for loading the SFLAG_INTEXTURE and SFLAG_INSRGBTEX surface locations.

D3DRS_ZBIAS is poorly defined, but it makes sense that the bias should be
format independent. Looking at application behaviour, it seems to include a
slope scale factor as well. This fixes a couple of regressions introduced by
96b758f7, although it was broken before as
well, just in a different way.