support REP (repeat) escape sequence
The sequence \e[Nb prints the last printed char N (more) times if it's
printable, and it's ignored after newline or other control chars.
This is Ecma-048/ANSI-X3.6 sequence and not DEC VT. It's supported by
xterm, and ncurses uses it when possible, e.g. when TERM is xterm* (and
with this commit also st*).
xterm supports only codepoints<=255, possibly due to internal limits.
We support any value/codepoint which was placed in a cell.
To test:
- tput rep 65 4 -> prints 'AAAA'
- printf "\342\225\246\033[4b" -> prints U+2566 1+4 times.
