Save target tilemap alpha in worldmap sector state

Saving the target tilemap alpha ensures that quitting a worldmap, while a tilemap fade is taking place, would not cause the tilemap to have an unintended alpha value on next enter.

src/object/tilemap.hpp

@@ -210,6 +210,8 @@ class TileMap final :
       target alpha. */
   float get_alpha() const;
 
+  float get_target_alpha() const { return m_alpha; }
+
   void set_tileset(const TileSet* new_tileset);
 
   const std::vector<uint32_t>& get_tiles() const { return m_tiles; }

src/worldmap/worldmap_state.cpp

@@ -361,7 +361,7 @@ WorldMapState::save_tilemap_visibility() const
     {
       sq_pushstring(vm.get_vm(), tilemap.get_name().c_str(), -1);
       sq_newtable(vm.get_vm());
-      vm.store_float("alpha", tilemap.get_alpha());
+      vm.store_float("alpha", tilemap.get_target_alpha());
       if (SQ_FAILED(sq_createslot(vm.get_vm(), -3)))
       {
         throw std::runtime_error("failed to create '" + m_worldmap.m_name + "' table entry");