them - previously we were ending up with four codepoints in one cell
which tmux believed to be width 2, but terminals considered width 4.
Secondly, invalidate cursor position before redrawing the cell when the
second codepoint is received, terminals vary in how they manage
backspace and cursor movement across these characters, so it is better
to use absolute rather than relative positioning. GitHub issue 4853.
position rather than keeping the last character from the input stream,
this is how most terminals work and fixes problems with displaying these
characters in vim. GitHub issue 3600.
