libvterm reads the first three sub-parameters of an SGR 38:2 or 48:2 sequence
as R:G:B, so a colour space id shifts the channels: the empty slot becomes red
via CSI_ARG_MISSING truncating to 255, and green and blue move over one.
Skip the colour space id when the colon-separated group holds more than three
arguments. The group length comes from CSI_ARG_HAS_MORE rather than the raw
argument count, so a following semicolon-separated parameter is not consumed.