restore `TB_DEFAULT==0` in all modes (#51)

restore `TB_DEFAULT==0` in all modes (#51)

this removes `TB_TRUECOLOR_DEFAULT` added in 846bba4b and goes back to
`TB_DEFAULT==0` in all modes. instead, hi-bit macros `TB_256_BLACK`
and `TB_TRUECOLOR_BLACK` are added which provide a way to emit black
in those modes.

turns out there were some subtle bugs associated with `TB_DEFAULT!=0`
(in addition to the bug identified in #51). the bugs were fixable but
required uglying up some code. zero as default in all modes is more
intuitive and easier to reason about (e.g., `memset(..., 0, ...)`).