### 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 (!tile_set) {
		return;
	}
	occluders.resize(tile_set->get_occlusion_layers_count());
	physics.resize(tile_set->get_physics_layers_count());
	for (int bit_index = 0; bit_index < 16; bit_index++) {
		if (terrain_set < 0 || terrain_peering_bits[bit_index] >= tile_set->get_terrains_count(terrain_set)) {
			terrain_peering_bits[bit_index] = -1;
		}
	}
	navigation.resize(tile_set->get_navigation_layers_count());
	
	custom_data.resize(tile_set->get_custom_data_layers_count());
	for (int i = 0; i < custom_data.size(); i++) {
		if (custom_data[i].get_type() != tile_set->get_custom_data_layer_type(i)) {
			Variant new_val;
			Callable::CallError error;
			if (Variant::can_convert(custom_data[i].get_type(), tile_set->get_custom_data_layer_type(i))) {
				const Variant *args[] = { &custom_data[i] };
				Variant::construct(tile_set->get_custom_data_layer_type(i), new_val, args, 1, error);
			} else {
				Variant::construct(tile_set->get_custom_data_layer_type(i), new_val, nullptr, 0, error);
			}
			custom_data.write[i] = new_val;
		}
	}
	notify_property_list_changed();
	emit_signal(SNAME("changed"));
}
void TileData::add_occlusion_layer(int p_to_pos) {