fix a transmute-to-force-floor time travel bug

js/tiletypes.js

@@ -18,7 +18,7 @@ function on_begin_force_floor(me, level) {
 
     me.type.on_arrive(me, level, actor);
     if (me.type.slide_mode) {
-        actor.slide_mode = me.type.slide_mode;
+        level._set_tile_prop(actor, 'slide_mode', me.type.slide_mode);
     }
 
     // Item bestowal
@@ -38,7 +38,7 @@ function on_begin_force_floor(me, level) {
     if (level.attempt_take(actor, item) && actor.ignores(me.type.name)) {
         // If they just picked up suction boots, they're no longer sliding
         // TODO this feels hacky, shouldn't the slide mode be erased some other way?
-        actor.slide_mode = null;
+        level._set_tile_prop(actor, 'slide_mode', null);
     }
 }