engine: change Polymost cvars around to be a little friendlier to users who want something other than the default lack of texture filtering, etc

New cvar hierarchy:

r_useindexedcolortextures overrides/disables r_anisotropy and r_usetileshades
r_texfilter overrides/disables r_useindexedcolortextures (and thus re-enables r_anisotropy and r_usetileshades)

This commit also fixes #235