Add XWayland support

With Cage becoming more popular since its mention on Phoronix and
therefore getting more use-cases than just my own project, add XWayland
support. The refactoring of 2cf40f7 makes this much easier. Note that
this is a no-cost addition for those of us not using XWayland as it is a
compile-time option that needs to be explicitly enabled by adding
`-Dxwayland=true` to your meson command.

seat.h

@@ -10,6 +10,9 @@
 #include "server.h"
 #include "view.h"
 
+#define DEFAULT_XCURSOR "left_ptr"
+#define XCURSOR_SIZE 24
+
 struct cg_seat {
 	struct wlr_seat *seat;
 	struct cg_server *server;