Use Xft.dpi for DPI if available.

fixes #2465

include/libi3.h

@@ -470,6 +470,13 @@ char *get_process_filename(const char *prefix);
  */
 char *get_exe_path(const char *argv0);
 
+/**
+ * Initialize the DPI setting.
+ * This will use the 'Xft.dpi' X resource if available and fall back to
+ * guessing the correct value otherwise.
+ */
+void init_dpi(void);
+
 /**
  * Convert a logical amount of pixels (e.g. 2 pixels on a “standard” 96 DPI
  * screen) to a corresponding amount of physical pixels on a standard or retina