TB_OUTPUT_256: Cannot use black / write CSI 38;5;0

[ljrk0]

The current implementation treats the value 0 (=TB_DEFAULT) for fg/bg special in write_sgr(). This leads to much unexpected behavior, for once, one cannot use the TB_* constants with this mode anymore.

Color	TB_OUTPUT_NORMAL	TB_OUTPUT_256
clear	0	0
black	1	-
red	2	1
...	...	...

What seems to be the most sensible but also most radical solution to me would be:

1. Make the TB_* constants line up with the actual values used ain SGR, ie. 0 for black etc.
2. Set TB_DEFAULT to a different magic value that's not used anywhere and catch that, if we are breaking, we could actually use -1 and use int16_t instead.
3. In fact, if we wanted to at some point support truecolor, we should perhaps even make it a bigger value.

Also I've found that the code uses the TB_BOLD constant for bg to encode "blinking", which should be documented.