### 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 (custom_data_layers_by_name.has(p_value)) {
		return custom_data_layers_by_name[p_value];
	} else {
		return -1;
	}
}
void TileSet::set_custom_data_layer_name(int p_layer_id, String p_value) {
	ERR_FAIL_INDEX(p_layer_id, custom_data_layers.size());
	
	if (!p_value.is_empty()) {
		for (int other_layer_id = 0; other_layer_id < get_custom_data_layers_count(); other_layer_id++) {
			if (other_layer_id != p_layer_id && get_custom_data_layer_name(other_layer_id) == p_value) {
				ERR_FAIL_MSG(vformat("There is already a custom property named %s", p_value));
			}
		}
	}
	if (p_value.is_empty() && custom_data_layers_by_name.has(p_value)) {
		custom_data_layers_by_name.erase(p_value);
	} else {
		custom_data_layers_by_name[p_value] = p_layer_id;
	}
	custom_data_layers.write[p_layer_id].name = p_value;
	emit_changed();
}
String TileSet::get_custom_data_layer_name(int p_layer_id) const {