backend/xcursor: Support a "blank" cursor type

We don't have enough Xlib code in mutter ...

Joking aside, it can be useful to make the cursor invisible
without hiding it, for example for replacing the actual cursor
with an actor in gnome-shell; the real cursor should still
update the focus surface in that case, and not sneak into
screenshots or -casts.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1244

src/meta/common.h

@@ -204,6 +204,7 @@ typedef enum
  * @META_CURSOR_POINTING_HAND: pointing hand
  * @META_CURSOR_CROSSHAIR: crosshair (action forbidden)
  * @META_CURSOR_IBEAM: I-beam (text input)
+ * @META_CURSOR_BLANK: Invisible cursor
  */
 typedef enum
 {
@@ -226,6 +227,7 @@ typedef enum
   META_CURSOR_POINTING_HAND,
   META_CURSOR_CROSSHAIR,
   META_CURSOR_IBEAM,
+  META_CURSOR_BLANK,
   META_CURSOR_LAST
 } MetaCursor;