### 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)

}
				return false;
			} else if (components[1].is_valid_int()) {
				int alternative_id = components[1].to_int();
				if (alternative_id != TileSetSource::INVALID_TILE_ALTERNATIVE) {
					
					if (!has_alternative_tile(coords, alternative_id)) {
						create_alternative_tile(coords, alternative_id);
					}
					if (!tiles[coords].alternatives.has(alternative_id)) {
						tiles[coords].alternatives[alternative_id] = memnew(TileData);
						tiles[coords].alternatives[alternative_id]->set_tile_set(tile_set);
						tiles[coords].alternatives[alternative_id]->set_allow_transform(alternative_id > 0);
						tiles[coords].alternatives_ids.push_back(alternative_id);
					}
					if (components.size() >= 3) {
						bool valid;
						tiles[coords].alternatives[alternative_id]->set(components[2], p_value, &valid);
						return valid;
					} else {
						
						return true;
					}
				}
			}
		}
	}
	return false;
}
bool TileSetAtlasSource::_get(const StringName &p_name, Variant &r_ret) const {