I've noticed this off and on for a while, and thought it was something
fishy with my shell dotfiles.
Tracing through I found that the final byte in the "Face with head
bandage" emoji 🤕 U+1F915 was being interpreted as the MW control
code and causing the vt parser to jump out of the OSC state.
The solution for this is to hook up proper UTF-8 processing in the
same way that it is applied in the ground state.
Since we don't have enough bits to introduce new state values (we're
pretty tightly packed in the 16 bits available), I've introduced a
memory of the state to which the utf8 parser needs to return once
a complete sequence is detected.
