### 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)

ClassDB::bind_method(D_METHOD("get_terrain_name", "terrain_set", "terrain_index"), &TileSet::get_terrain_name);
	ClassDB::bind_method(D_METHOD("set_terrain_color", "terrain_set", "terrain_index", "color"), &TileSet::set_terrain_color);
	ClassDB::bind_method(D_METHOD("get_terrain_color", "terrain_set", "terrain_index"), &TileSet::get_terrain_color);
	
	ClassDB::bind_method(D_METHOD("get_navigation_layers_count"), &TileSet::get_navigation_layers_count);
	ClassDB::bind_method(D_METHOD("add_navigation_layer", "to_position"), &TileSet::add_navigation_layer, DEFVAL(-1));
	ClassDB::bind_method(D_METHOD("move_navigation_layer", "layer_index", "to_position"), &TileSet::move_navigation_layer);
	ClassDB::bind_method(D_METHOD("remove_navigation_layer", "layer_index"), &TileSet::remove_navigation_layer);
	ClassDB::bind_method(D_METHOD("set_navigation_layer_layers", "layer_index", "layers"), &TileSet::set_navigation_layer_layers);
	ClassDB::bind_method(D_METHOD("get_navigation_layer_layers", "layer_index"), &TileSet::get_navigation_layer_layers);
	ClassDB::bind_method(D_METHOD("set_navigation_layer_layer_value", "layer_index", "layer_number", "value"), &TileSet::set_navigation_layer_layer_value);
	ClassDB::bind_method(D_METHOD("get_navigation_layer_layer_value", "layer_index", "layer_number"), &TileSet::get_navigation_layer_layer_value);
	
	ClassDB::bind_method(D_METHOD("get_custom_data_layers_count"), &TileSet::get_custom_data_layers_count);
	ClassDB::bind_method(D_METHOD("add_custom_data_layer", "to_position"), &TileSet::add_custom_data_layer, DEFVAL(-1));
	ClassDB::bind_method(D_METHOD("move_custom_data_layer", "layer_index", "to_position"), &TileSet::move_custom_data_layer);
	ClassDB::bind_method(D_METHOD("remove_custom_data_layer", "layer_index"), &TileSet::remove_custom_data_layer);
	ClassDB::bind_method(D_METHOD("get_custom_data_layer_by_name", "layer_name"), &TileSet::get_custom_data_layer_by_name);
	ClassDB::bind_method(D_METHOD("set_custom_data_layer_name", "layer_index", "layer_name"), &TileSet::set_custom_data_layer_name);
	ClassDB::bind_method(D_METHOD("get_custom_data_layer_name", "layer_index"), &TileSet::get_custom_data_layer_name);
	ClassDB::bind_method(D_METHOD("set_custom_data_layer_type", "layer_index", "layer_type"), &TileSet::set_custom_data_layer_type);
	ClassDB::bind_method(D_METHOD("get_custom_data_layer_type", "layer_index"), &TileSet::get_custom_data_layer_type);
	
	ClassDB::bind_method(D_METHOD("set_source_level_tile_proxy", "source_from", "source_to"), &TileSet::set_source_level_tile_proxy);
	ClassDB::bind_method(D_METHOD("get_source_level_tile_proxy", "source_from"), &TileSet::get_source_level_tile_proxy);
	ClassDB::bind_method(D_METHOD("has_source_level_tile_proxy", "source_from"), &TileSet::has_source_level_tile_proxy);
	ClassDB::bind_method(D_METHOD("remove_source_level_tile_proxy", "source_from"), &TileSet::remove_source_level_tile_proxy);
	ClassDB::bind_method(D_METHOD("set_coords_level_tile_proxy", "p_source_from", "coords_from", "source_to", "coords_to"), &TileSet::set_coords_level_tile_proxy);
	ClassDB::bind_method(D_METHOD("get_coords_level_tile_proxy", "source_from", "coords_from"), &TileSet::get_coords_level_tile_proxy);
	ClassDB::bind_method(D_METHOD("has_coords_level_tile_proxy", "source_from", "coords_from"), &TileSet::has_coords_level_tile_proxy);