### 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)

ERR_FAIL_INDEX_V(p_terrain_set, terrain_sets.size(), RBSet<TileMapCell>());
	_update_terrains_cache();
	return per_terrain_pattern_tiles[p_terrain_set][p_terrain_tile_pattern];
}
TileMapCell TileSet::get_random_tile_from_terrains_pattern(int p_terrain_set, TileSet::TerrainsPattern p_terrain_tile_pattern) {
	ERR_FAIL_INDEX_V(p_terrain_set, terrain_sets.size(), TileMapCell());
	_update_terrains_cache();
	
	double sum = 0.0;
	RBSet<TileMapCell> set = per_terrain_pattern_tiles[p_terrain_set][p_terrain_tile_pattern];
	for (const TileMapCell &E : set) {
		if (E.source_id >= 0) {
			Ref<TileSetSource> source = sources[E.source_id];
			Ref<TileSetAtlasSource> atlas_source = source;
			if (atlas_source.is_valid()) {
				TileData *tile_data = atlas_source->get_tile_data(E.get_atlas_coords(), E.alternative_tile);
				sum += tile_data->get_probability();
			} else {
				sum += 1.0;
			}
		} else {
			sum += 1.0;
		}
	}
	
	double count = 0.0;