only LOG() the DPI when it changes, DLOG() it otherwise (Thanks lkraav)

This avoids flooding stdout every time some text (e.g. a window
decoration) is drawn, yet leaves the message in place when it’s actually
relevant (upon DPI changes).

fixes #1115

i3-nagbar/main.c

@@ -61,7 +61,7 @@ xcb_connection_t *conn;
 xcb_screen_t *root_screen;
 
 /*
- * Having verboselog() and errorlog() is necessary when using libi3.
+ * Having verboselog(), errorlog() and debuglog() is necessary when using libi3.
  *
  */
 void verboselog(char *fmt, ...) {
@@ -80,6 +80,9 @@ void errorlog(char *fmt, ...) {
     va_end(args);
 }
 
+void debuglog(char *fmt, ...) {
+}
+
 /*
  * Starts the given application by passing it through a shell. We use double fork
  * to avoid zombie processes. As the started application’s parent exits (immediately),