More documentation, cleanups, and a cache for get_colorpixel()

include/util.h

@@ -26,6 +26,7 @@ int min(int a, int b);
 int max(int a, int b);
 void die(char *fmt, ...);
 void *smalloc(size_t size);
+void *scalloc(size_t size);
 char *sstrdup(const char *str);
 void start_application(const char *command);
 void check_error(xcb_connection_t *connection, xcb_void_cookie_t cookie, char *err_message);