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

src/con.c

@@ -47,7 +47,7 @@ Con *con_new_skeleton(Con *parent, i3Window *window) {
         new->depth = window->depth;
         new->window->aspect_ratio = 0.0;
     } else {
-        new->depth = XCB_COPY_FROM_PARENT;
+        new->depth = root_depth;
     }
     DLOG("opening window\n");