### 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 (polygon_index >= physics[layer_index].polygons.size()) {
					physics.write[layer_index].polygons.resize(polygon_index + 1);
				}
			}
			if (components[2] == "points") {
				Vector<Vector2> polygon = p_value;
				set_collision_polygon_points(layer_index, polygon_index, polygon);
				return true;
			} else if (components[2] == "one_way") {
				set_collision_polygon_one_way(layer_index, polygon_index, p_value);
				return true;
			} else if (components[2] == "one_way_margin") {
				set_collision_polygon_one_way_margin(layer_index, polygon_index, p_value);
				return true;
			}
		}
	} else if (components.size() == 2 && components[0].begins_with("navigation_layer_") && components[0].trim_prefix("navigation_layer_").is_valid_int()) {
		
		int layer_index = components[0].trim_prefix("navigation_layer_").to_int();
		ERR_FAIL_COND_V(layer_index < 0, false);
		if (components[1] == "polygon") {
			Ref<NavigationPolygon> polygon = p_value;
			if (layer_index >= navigation.size()) {
				if (tile_set) {
					return false;
				} else {
					navigation.resize(layer_index + 1);
				}
			}