engine.c: instead of fogpal post-tweaks (last 2 commits), normalize by numshades-1.

That is, for every for palette (now, even ones created by the user) the last
shade is the (or rather, a color close to the) full fog color by construction.
In loadpalette(), reject a PALETTE.DAT with only one shade level.

git-svn-id: http://svn.eduke32.com/eduke32@4458 1a8010ca-5511-0410-912e-c29ae57300e0
Polymost: make r_usetileshades 2 the default.

This mode does not take a sector's visibility values into account for uniform
darkening of a texture. It is more faithful to classic with respect to
fog/visibility *distances*, but may be less so as far as the "tint" of the
texture due to visibility attenuation is concerned.

git-svn-id: http://svn.eduke32.com/eduke32@4459 1a8010ca-5511-0410-912e-c29ae57300e0