inputw: improve correctness and startup performance

inputw: improve correctness and startup performance

a massive amount of time inside readstdin() is spent trying to get the
max input width and then put it into inputw, only for it to get clamped
down to mw/3 inside setup().

it makes more sense to calculate inputw inside setup() once we have mw
available. similar to the last patch, i see noticeable startup
performance improvement:

before -> after
160ms  -> 60ms

additionally this will take fallback fonts into account compared to the
previous version, so it's not only more performant but also more correct.