backends: Replace MetaCursorSprite::prepare-at with in-place function

Since this signal is in a hot path during input handling, it makes sense
not to have this be a signal at all, currently most of the time spent in
it is in GLib signal machinery itself.

Replace it with a function/user data pair that are set on the sprite
itself. Only the places that create an sprite are interested in hooking
one ::prepare-at behavior per sprite, so we can do with a single pair.

This makes meta_cursor_sprite_prepare_at() inexpensive enough.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1915>

src/core/display.c

@@ -1729,11 +1729,9 @@ static void
 manage_root_cursor_sprite_scale (MetaDisplay             *display,
                                  MetaCursorSpriteXcursor *sprite_xcursor)
 {
-  g_signal_connect_object (sprite_xcursor,
-                           "prepare-at",
-                           G_CALLBACK (root_cursor_prepare_at),
-                           display,
-                           0);
+  meta_cursor_sprite_set_prepare_func (META_CURSOR_SPRITE (sprite_xcursor),
+                                       (MetaCursorPrepareFunc) root_cursor_prepare_at,
+                                       display);
 }
 
 void