libi3: add logical_px() for Retina display support

include/libi3.h

@@ -381,3 +381,11 @@ char *get_process_filename(const char *prefix);
  * Returned value must be freed by the caller.
  */
 char *get_exe_path(const char *argv0);
+
+/**
+ * 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
+ * screen, e.g. 5 pixels on a 227 DPI MacBook Pro 13" Retina screen.
+ *
+ */
+int logical_px(const int logical);

libi3/dpi.c

@@ -0,0 +1,16 @@
+#include "libi3.h"
+#include <math.h>
+
+extern xcb_screen_t *root_screen;
+
+/*
+ * 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
+ * screen, e.g. 5 pixels on a 227 DPI MacBook Pro 13" Retina screen.
+ *
+ */
+int logical_px(const int logical) {
+    const double dpi = (double)root_screen->height_in_pixels * 25.4 /
+                       (double)root_screen->height_in_millimeters;
+	return ceil((dpi / 96.0) * logical);
+}