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

i3bar/src/main.c

@@ -18,7 +18,7 @@
 #include "common.h"
 
 /*
- * Having verboselog() and errorlog() is necessary when using libi3.
+ * Having verboselog(), errorlog() and debuglog() is necessary when using libi3.
  *
  */
 void verboselog(char *fmt, ...) {
@@ -37,6 +37,9 @@ void errorlog(char *fmt, ...) {
     va_end(args);
 }
 
+void debuglog(char *fmt, ...) {
+}
+
 /*
  * Glob path, i.e. expand ~
  *