sync updated drw code from dmenu

important:
- drw_rect: didn't use w and h, change the dwm code accordingly.
- drw_text: text is NULL is not allowed, use drw_rect().

util.h

@@ -5,3 +5,4 @@
 #define BETWEEN(X, A, B)        ((A) <= (X) && (X) <= (B))
 
 void die(const char *errstr, ...);
+void *ecalloc(size_t, size_t);