deprecate setting colour palette for > 16 colours
vte_terminal_set_colors() says it supports @palette_size of 0, 8, 16, 232 or 256.
AFAIK setting colours > 16 is rarely done if ever, and also applications have a reasonable expectation that when they use the 6,6,6 colour cube and the greyscale ramp colours, they get the exact colours, not something else.
Therefore I'd like to deprecate setting colours outside the 16-colour palette (8 normal plus 8 bright colours).