Add overlay to TileMapLayerEditor specifically for missing tile warning textures

[radioactivejackal1414]

What problem(s) does this PR solve?

• Closes Missing tile warnings in TileMap dock display with wrong texture filter #123776

I added a new Control called tile_warning_overlay which is a child of custom_overlay, so that the warnings have their own overlay which is always set to a Linear filter.

Additional information

Related: #123692

[gabcont]

I just tested your solution and it works as expected, the split of custom_overlay control node into two specific nodes is, at least for me, the right move.

However there are some issues I have:

1. Why is tile_warning_overlay a child of custom_overlay? That hierarchy does not makes sense to me, I would think they are siblings, which I think is a better way of structuring this.
2. There is some duplication of code at the start of both overlays' _draw functions, it is not much but that is called every frame. However, I dont see a way to optimize it.
3. I think custom_overlay is opaque naming that hurts readibiity of this file. I would suggest preview_overlay, although it also handles grid and other visuals, so maybe tile_placing_overlay or similar, as it really only handles visual elements for previewing tiles and their position.

Overall it looks fine to me, although I strongly suggest the renaming.

[radioactivejackal1414]

Thanks for reviewing this @gabcont !

I've updated this based on your feedback:

• tile_warning_overlay is now siblings with custom_overlay.
• I agree that the naming of custom_overlay is not ideal. Since I'm splitting it into 2 controls I might as well give it a better name. I went with tile_grid_overlay since it's used for more than just previews.
• I don't see a simple solution to avoid the duplicated code (but it bothered me too). I wanted the simplest change and the duplicated code felt like a small inconvenience.
• I also added a block (if (!tile_warning_overlay) {) that matches the logic for custom_overlay, which I was missing before.
• I added TEXTURE_REPEAT_ENABLED since it should be there too. The repeating pattern wasn't smooth before:

Before: (TEXTURE_REPEAT_DISABLED)

After: (TEXTURE_REPEAT_ENABLED)