significantly improve performance on large strings
this replaces inefficient pattern of `MIN(TEXTW(..), n)` with
drw_fontset_getwidth_clamp() instead, which is far more efficient when
we only want up to a certain width.
dumping a decently sized (unicode) emoji file into dmenu, I see the
startup time drop significantly with this patch.
before -> after
360ms -> 160ms
this should also noticeably improve input latency (responsiveness) given
that calcoffsets() and drawmenu() are pretty hot functions.