### 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)

ERR_FAIL_COND_MSG(!tiles.has(p_atlas_coords), vformat("TileSetAtlasSource has no tile at %s.", String(p_atlas_coords)));
	
	_clear_coords_mapping_cache(p_atlas_coords);
	
	for (const KeyValue<int, TileData *> &E_tile_data : tiles[p_atlas_coords].alternatives) {
		memdelete(E_tile_data.value);
	}
	
	tiles.erase(p_atlas_coords);
	tiles_ids.erase(p_atlas_coords);
	tiles_ids.sort();
	_queue_update_padded_texture();
	emit_signal(SNAME("changed"));
}
bool TileSetAtlasSource::has_tile(Vector2i p_atlas_coords) const {