this patch makes some non-trivial changes, which significantly improves
the performance of drawing large strings as well as fixes any issues
regarding the printing of the ellipsis when string gets truncated.

* performance:

before there were two O(n) loops, one which finds how long we can go
without changing font, and the second loop would (incorrectly) truncate
the string if it's too big.

this patch merges the overflow calculation into the first loop and exits
out when overflow is detected. when dumping lots of emojies into dmenu,
i see some noticeable startup time improvement:

before -> after
460ms  -> 360ms

input latency when scrolling up/down is also noticeably better and can
be tested with the following:

	for _ in $(seq 20); do
		cat /dev/urandom | base64 | tr -d '\n' | head -c 1000000
	echo
	done | ./dmenu -l 10

* correctness:

the previous version would incorrectly assumed single byte chars and
would overwrite them with '.' , this caused a whole bunch of obvious
problems, including the ellipsis not getting rendered if then font
changed.

in addition to exiting out when we detect overflow, this patch also
keeps track of the last x-position where the ellipsis would fit. if we
detect overflow, we simply make a recursing call to drw_text() at the
ellipsis_x position and overwrite what was there.

so now the ellipsis will always be printed properly, regardless of
weather the font changes or if the string is single byte char or not.

the idea of rendering the ellipsis on top incase of overflow was
from Bakkeby <bakkeby@gmail.com>, thanks! however the original patch had
some issues incorrectly truncating the prompt (-p flag) and cutting off
emojies. those have been fixed in here.