The current code treated SH_LUT_TEXTURE like SH_LUT_LINEAR for no good
reason (which was also wrong since d178fd2). Furthermore, it claimed
SH_LUT_LINEAR used integer addressing, which is not the case.

Make this more consistent by treating *all* LUT methods as integer
addressing except for SH_LUT_LINEAR (which is the only sane exception).

Fixes the dither bugs since d178fd2, and also a hypothetical bug if
SH_LUT_AUTO decided to use uniform variables as the LUT method.