### 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)

Transform2D xform;
							xform = flip_h ? xform.scaled(Size2(-1, 1)) : xform;
							xform = flip_v ? xform.scaled(Size2(1, -1)) : xform;
							xform = transpose ? Transform2D(xform[1], xform[0], Vector2()) : xform;
							int alternative_tile = 0;
							if (!atlas_source->has_tile(coords)) {
								atlas_source->create_tile(coords);
							} else {
								alternative_tile = atlas_source->create_alternative_tile(coords);
							}
							
							Array key_array;
							key_array.push_back(coords);
							key_array.push_back(flip_h);
							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_ATLAS_TILE;
							TileData *tile_data = atlas_source->get_tile_data(coords, alternative_tile);
							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);