r300: Cleanup LodBias support

r300: Cleanup LodBias support

. There is both a per-texture unit and a per-texture object (at least for
OpenGL 1.4); this should now be supported properly.
. The LOD bias calculation in r300_state has been simplified and corrected
  (need to multiply by 32 instead of 31, and ensure clamping)
. do not clamp LOD bias in TexEnv, as that behaviour conflicts with what
  the spec says
. set Const.MaxTextureLodBias properly
. remove the no_neg_lod_bias property; if somebody can explain what
  it's good for, we can add it back in, but according to Google, nobody
  seems to use it
. removed some dead code and unused variables