sync drw.{c,h} from dmenu

- drw: minor improvement to the nomatches cache
- overhaul utf8decoding and render invalid utf8 sequences as U+FFFD.

Thanks NRK for these improvements!

util.h

@@ -3,6 +3,7 @@
 #define MAX(A, B)               ((A) > (B) ? (A) : (B))
 #define MIN(A, B)               ((A) < (B) ? (A) : (B))
 #define BETWEEN(X, A, B)        ((A) <= (X) && (X) <= (B))
+#define LENGTH(X)               (sizeof (X) / sizeof (X)[0])
 
 void die(const char *fmt, ...);
 void *ecalloc(size_t nmemb, size_t size);