ssd: Allocate struct ssd and struct ssd_hover_state separately

- Store a pointer to the `struct view` in `struct ssd`
- Pass `struct ssd *` instead of `struct view *` to ssd functions
- Add `ssd_get_margin()` convenience function

src/server.c

@@ -221,6 +221,8 @@ server_init(struct server *server)
 	wl_list_init(&server->views);
 	wl_list_init(&server->unmanaged_surfaces);
 
+	server->ssd_hover_state = ssd_hover_state_new();
+
 	server->scene = wlr_scene_create();
 	if (!server->scene) {
 		wlr_log(WLR_ERROR, "unable to create scene");