### 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)

key_array.push_back(flip_v);
					key_array.push_back(transpose);
					Array value_array;
					value_array.push_back(source_id);
					value_array.push_back(coords);
					value_array.push_back(alternative_tile);
					if (!compatibility_tilemap_mapping.has(E.key)) {
						compatibility_tilemap_mapping[E.key] = RBMap<Array, Array>();
					}
					compatibility_tilemap_mapping[E.key][key_array] = value_array;
					compatibility_tilemap_mapping_tile_modes[E.key] = COMPATIBILITY_TILE_MODE_SINGLE_TILE;
					TileData *tile_data = atlas_source->get_tile_data(coords, alternative_tile);
					ERR_CONTINUE(!tile_data);
					tile_data->set_flip_h(flip_h);
					tile_data->set_flip_v(flip_v);
					tile_data->set_transpose(transpose);
					tile_data->set_material(ctd->material);
					tile_data->set_modulate(ctd->modulate);
					tile_data->set_z_index(ctd->z_index);
					if (ctd->occluder.is_valid()) {
						if (get_occlusion_layers_count() < 1) {
							add_occlusion_layer();
						};
						Ref<OccluderPolygon2D> occluder = ctd->occluder->duplicate();
						Vector<Vector2> polygon = ctd->occluder->get_polygon();
						for (int index = 0; index < polygon.size(); index++) {
							polygon.write[index] = xform.xform(polygon[index] - ctd->region.get_size() / 2.0);
						}
						occluder->set_polygon(polygon);
						tile_data->set_occluder(0, occluder);