This corrects an issue where the mode byte of the DCS sequence was
discarded from the DcsHook, making it impossible to know what sequence
is being activated.
So far this hasn't come up as these sequences are relatively rare,
but in looking at sixel parsing I noticed the error.
These are used in the default Fedora 31 bash profile, so it seems
worth handling even if they are a bit amgiguously defined.
Closes: https://github.com/wez/wezterm/issues/86
