### 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)

tile_data->set_z_index(ctd->autotile_z_index_map[coords]);
							}
							
							if (ctd->shapes.size() > 0) {
								if (get_physics_layers_count() < 1) {
									add_physics_layer();
								}
							}
							for (int k = 0; k < ctd->shapes.size(); k++) {
								CompatibilityShapeData csd = ctd->shapes[k];
								if (csd.autotile_coords == coords) {
									Ref<ConvexPolygonShape2D> convex_shape = csd.shape; 
									if (convex_shape.is_valid()) {
										Vector<Vector2> polygon = convex_shape->get_points();
										for (int point_index = 0; point_index < polygon.size(); point_index++) {
											polygon.write[point_index] = xform.xform(csd.transform.xform(polygon[point_index]) - ctd->autotile_tile_size / 2.0);
										}
										tile_data->set_collision_polygons_count(0, tile_data->get_collision_polygons_count(0) + 1);
										int index = tile_data->get_collision_polygons_count(0) - 1;
										tile_data->set_collision_polygon_one_way(0, index, csd.one_way);
										tile_data->set_collision_polygon_one_way_margin(0, index, csd.one_way_margin);
										tile_data->set_collision_polygon_points(0, index, polygon);
									}
								}
							}