Sign crashes when loaded due to out of bounds text on the image (debug only assertion)

see attached world to reproduce

```
Exception in thread "Scene Manager" java.lang.AssertionError
at se.llbit.chunky.resources.PalettizedBitmapImage.setPixel(PalettizedBitmapImage.java:37)
at se.llbit.chunky.resources.SignTexture.(SignTexture.java:100)
at se.llbit.chunky.entity.WallSignEntity.(WallSignEntity.java:103)
at se.llbit.chunky.entity.WallSignEntity.(WallSignEntity.java:95)
at se.llbit.chunky.block.WallSign.toBlockEntity(WallSign.java:48)
at se.llbit.chunky.renderer.scene.Scene.loadChunks(Scene.java:1314)
at se.llbit.chunky.renderer.scene.SynchronousSceneManager.loadFreshChunks(SynchronousSceneManager.java:230)
at se.llbit.chunky.renderer.scene.AsynchronousSceneManager.lambda$loadFreshChunks$4(AsynchronousSceneManager.java:161)
at se.llbit.chunky.renderer.scene.AsynchronousSceneManager.run(AsynchronousSceneManager.java:82)
```

[crashy sign.zip](https://github.com/chunky-dev/chunky/files/8664689/crashy.sign.zip)