wayland: set default cursor size to 24

Set it to 24 if it couldn't be read frome $XCURSOR_SIZE for some reason.
Since it seems to be the default value for most distros/DE.

video/out/wayland_common.c

@@ -1429,7 +1429,7 @@ static int spawn_cursor(struct vo_wayland_state *wl)
 
     const char *xcursor_theme = getenv("XCURSOR_THEME");
     const char *size_str = getenv("XCURSOR_SIZE");
-    int size = 32;
+    int size = 24;
     if (size_str != NULL) {
         errno = 0;
         char *end;