### Godot version

5ee983188de97ae027f9b9c1443438063f708a7e

### System information

Windows 11

### Issue description

Godot editor will throw an error and fail to generate preview icons for terrain tiles which use a base image which is not RGBA8 format.
`core\io\image.cpp:2788 - Condition "format != p_src->format" is true.`

### Steps to reproduce

- Import a non-RGBA8 image into Godot.
- Create a TileMap
- Create an associated TileSet
- Create a terrain for this tileset
- Add the image as an atlas
- Go to the paint tab
- Paint the terrain on the TileSet
- An error will occur any time terrains are edited in the TileSet editor or selected in the TileMap editor terrain tab.

### Minimal reproduction project

[tileset_terrain_bug.zip](https://github.com/godotengine/godot/files/13264213/tileset_terrain_bug.zip)

if (source->has_tile(p_coords_from) && source->has_alternative_tile(p_coords_from, p_alternative_from)) {
			return from;
		}
	}
	
	if (alternative_level_proxies.has(from)) {
		return alternative_level_proxies[from].duplicate();
	}
	
	from.pop_back();
	if (coords_level_proxies.has(from)) {
		Array output = coords_level_proxies[from].duplicate();
		output.push_back(p_alternative_from);
		return output;
	}
	
	if (source_level_proxies.has(p_source_from)) {
		Array output;
		output.push_back(source_level_proxies[p_source_from]);
		output.push_back(p_coords_from);
		output.push_back(p_alternative_from);
		return output;
	}
	Array output;
	output.push_back(p_source_from);
	output.push_back(p_coords_from);
	output.push_back(p_alternative_from);
	return output;
}
void TileSet::cleanup_invalid_tile_proxies() {