This patch fixes multiple bugs in countprompt.

1. Height off by one in the presence of meta characters at the end of the line.

The following prompt has height 2 but countprompt used to return 3.

    PROMPT="${(pl.$COLUMNS..-.)}%f"$'\n'

You can observe the effects of the bug by hitting esc-x followed by
reset-prompt.

2. Width off by one when a line is broken in the middle of a wide character.

Assuming COLUMNS=79, the following prompt has width 2 but countprompt
used to return 0. Note that \u3050 has width 2.

    PROMPT="${(pl.40..\u3050.)}" zsh -df

Press ctrl-r or type ls<tab> to observe the effects of the bug.

3. Width off by 1-7 when a line is broken in the middle of a tab.

Assuming COLUMNS=79, the following prompt has width 1 but countprompt
used to return 0.

    PROMPT="${(pl.10..\t.)}" zsh -df

Press ctrl-r or type ls<tab> to observe the effects of the bug.

On github: https://github.com/zsh-users/zsh/compare/master...romkatv:countprompt-fix.

I've only tested it on one terminal. Before I test it on a dozen of
different terminals, is there anything I'm missing? Anything I should
be on lookout for?

Roman.