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-config-wizard/main.c

@@ -420,7 +420,7 @@ static char *rewrite_binding(const char *input) {
 
 
 /*
- * Having verboselog() and errorlog() is necessary when using libi3.
+ * Having verboselog(), errorlog() and debuglog() is necessary when using libi3.
  *
  */
 void verboselog(char *fmt, ...) {
@@ -439,6 +439,9 @@ void errorlog(char *fmt, ...) {
     va_end(args);
 }
 
+void debuglog(char *fmt, ...) {
+}
+
 /*
  * This function resolves ~ in pathnames.
  * It may resolve wildcards in the first part of the path, but if no match