Fixes#1343
If a wide character is found at the right edge of a terminal screen and
can't be printed without wrapping, the wide character is ignored. This
matches xterm behavior.
The one funky behavior is with grapheme clustering enabled and VS16
emoji. For VS16, we act as if VS16 was never received. This is not
specified in any way and no other terminal handles this correctly at the
time of authoring this so we're just making this up because it seems
most sensible.
8b23d5c9ab