256 colorcube - follow the standards that need a set of custom colors

my xterm-256tst does seem to demand a specialized colorcube and
according to T846 this is the case elsewhere, so change colorcube to
match, as opposed to one that is more color efficient.
Fix two small warnings

Reviewers: billiob, raster

Differential Revision: https://phab.enlightenment.org/D496