libdraw: fix zero stringwidth() bug

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.

sys/src/libdraw/stringwidth.c

@@ -39,7 +39,7 @@ _stringnwidth(Font *f, char *s, Rune *r, int len)
 			}
 		}
 		if((n = cachechars(f, sptr, rptr, cbuf, max, &wid, &subfontname)) <= 0){
-			if(subfontname){
+			if(n == 0){
 				if(++try > 10)
 					break;
 				continue;