stringwidth() and string() sometimes failed spuriously due
to the wrong assumption that cachechars() will only fail
when a different subfont is needed. in fact, cachechars()
can fail for other reasons like when it resizes the fontcache
(or fails todo so).
theres also the case when loadchar() is unable to translate
a character and returns 0. this case needs to be differentiated
from such temporary conditions like fontcache resize or subfont
load to stop the retry loop in string() and stringwidth().
now cachechars() returns -1 to indicate that it cannot
proceed and we test this in string() and stringwidth()
to skip over untranslatable characters to make progress
instead of retrying.
cachechars() used to skip over characters on its own when
loadchar() could not find the character or a PJW replacement.
this resulted in wrong width calculation. now we just return
and handle the case inside _string and _stringwidth.
fix subfont leak in stringwidth()
remove annoying prints in stringwidth()
