Use 32-bit visual by default if available.

With this patch, we use 32-bit visuals per default whenever it is
available. Otherwise, we fall back to the actual root window's
depth, which will typically be 24-bit.

Before this patch, we already used 32-bit depth for containers with
a window that uses 32-bit. However, this means that we didn't use
32-bit for split parent containers on which decoration is drawn.
For 32-bit windows using transparency, this caused a graphical glitch
because the decoration pixmap behind it would show through. This
behavior is fixed with this change.

relates to #1278

include/xcb.h

@@ -139,6 +139,11 @@ void xcb_set_root_cursor(int cursor);
  *
  */
 uint16_t get_visual_depth(xcb_visualid_t visual_id);
+
+/**
+ * Get visual type specified by visualid
+ *
+ */
 xcb_visualtype_t *get_visualtype_by_id(xcb_visualid_t visual_id);
 
 /**