Builtin vars with special behavior are often not handled properly

[ike709]

Examples of vars that aren't handled properly (the real list is a lot longer):

• [x] overlays - Fixed in https://github.com/wixoaGit/OpenDream/pull/555
• [x] underlays - Fixed in https://github.com/wixoaGit/OpenDream/pull/555
• [x] invisibility - Fixed in https://github.com/wixoaGit/OpenDream/pull/555
• [ ] contents

These should be pretty easy to identify by just checking OnVariableSet(). If it has custom set logic and doesn't directly manipulate its own value, it's probably handled wrong.