small update

• the main skybox was overwhelmingly white at the bottom which threw off the overall aesthetic when looking down, so I've mirrored a section of it
• a few visual tweaks to sokoban parts
• reverted the level-entry platforms to use 3D locks and stars as the vector images weren't working well for those. I'll continue to refine the game-wide balance between using 3D props vs 2D vector images