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-input/main.c

@@ -57,7 +57,7 @@ xcb_screen_t *root_screen;
 static xcb_get_input_focus_cookie_t focus_cookie;
 
 /*
- * Having verboselog() and errorlog() is necessary when using libi3.
+ * Having verboselog(), errorlog() and debuglog() is necessary when using libi3.
  *
  */
 void verboselog(char *fmt, ...) {
@@ -76,6 +76,9 @@ void errorlog(char *fmt, ...) {
     va_end(args);
 }
 
+void debuglog(char *fmt, ...) {
+}
+
 /*
  * Restores the X11 input focus to whereever it was before.
  * This is necessary because i3-input’s window has override_redirect=1