tui: Do not optimize left motion at the right margin.

From observation, there are several different possible behaviours:

1. Deferred wrap like a real DEC VT.  The cursor stays visible in the last
   column, and CUB is calculated relative to that column.
   Examples: xterm, Unicode rxvt, PuTTY, nosh console-terminal-emulator,
   FreeBSD kernel's built-in emulator, Linux's built-in emulator
2. Deferred wrap like a real DEC VT.  CUB is calculated relative to the last
   column.  But the cursor is invisible.
   Examples: emulators using newer libvte
3. Non-deferred wrap.  The cursor has already wrapped to the next line and CUB
   does not wrap back.
   Examples: cygwin, Interix
4. Non-deferred wrap that acts like deferred wrap.  The cursor has already
   visibly wrapped to the next line, but CUB can wrap back around the left
   margin.
   Examples: Konsole
5. Deferred wrap with visibly out of bounds cursor.  The cursor visibly moves
   outwith the screen boundaries.  CUB is calculated relative to a cursor
   column that has overflowed the end of the screen grid array.
   Examples: iTerm2
6. Deferred wrap with invisibly out of bounds cursor.  CUB is calculated
   relative to a cursor column that has overflowed the end of the screen grid
   array.  And the cursor is invisible.
   Examples: emulators using older libvte

In many cases, nvim does not have enough information to know which behaviour
the terminal will exhibit, and thus the correct amount of CUB to issue.

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