Decals get overlayed into the void, and become invisible.

### Testmerges

N/A

### Round ID

N/A

### Description of the bug

Decals with offsets that make them go into closed turfs will vanish.
Due to https://github.com/cmss13-devs/cmss13/blob/df7f6784119efc6c17aaff5e6b60b66a6c26f950/code/game/objects/effects/decals/decal.dm#L17-L25

This occurs:

If we change the -y offset that's making it go into the windows turf we can actually see it.
The other two are visible because they're overlayed on the floor under the door.

### What's the difference with what should have happened?

It should be visible and not either poof into nonexistence or get overlayed onto something that can't be viewed.

### How do we reproduce this bug?

1. This is probably an issue for more maps than I'm aware of but if #11008 is on TM you can easily see that most decals that are north of walls are affected because you can't see them
2. But you can see them in StrongDMM
3. Cry
...

### Issue Bingo

- [x] Issue could be reproduced at least once
- [x] Issue happened in a recent (less than 7 days ago) round
- [x] Couldn't find an existing issue about this (https://github.com/cmss13-devs/cmss13/issues)