[Mlterm-dev-en] True true-color support

[Yuri D'E. <wa...@th...>]

vt_color_mode/--vtcolor accepts "true" to set true color mode, however
it's limited to a 512-color palette internally:

https://sourceforge.net/p/mlterm/bugs/74/

console_sixel_colors however does seem to support real true-color mode.

I wonder why this limitation exists?
I guess internally all vt operations operate on a color index?