### 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_modulate(ctd->modulate);
							tile_data->set_z_index(ctd->z_index);
							if (ctd->autotile_occluder_map.has(coords)) {
								if (get_occlusion_layers_count() < 1) {
									add_occlusion_layer();
								}
								Ref<OccluderPolygon2D> occluder = ctd->autotile_occluder_map[coords]->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);
							}
							if (ctd->autotile_navpoly_map.has(coords)) {
								if (get_navigation_layers_count() < 1) {
									add_navigation_layer();
								}
								Ref<NavigationPolygon> navigation = ctd->autotile_navpoly_map[coords]->duplicate();
								Vector<Vector2> vertices = navigation->get_vertices();
								for (int index = 0; index < vertices.size(); index++) {
									vertices.write[index] = xform.xform(vertices[index] - ctd->region.get_size() / 2.0);
								}
								navigation->set_vertices(vertices);
								tile_data->set_navigation_polygon(0, navigation);
							}
							if (ctd->autotile_priority_map.has(coords)) {
								tile_data->set_probability(ctd->autotile_priority_map[coords]);
							}
							if (ctd->autotile_z_index_map.has(coords)) {