Trailing black pixels for RAW with zoom

When specifying zoom, the raw output produces trailing black pixels because of a faulty calculation in the output writer.

To fix: on `Ln 43` of `raw_output_writer.cpp` change 

```cpp
write(white, zoom_ * 2 * size.width + 1);
```
to
```cpp
write(white, zoom_ * (2 * size.width + 1));
```

Sorry, didn't feel like creating a PR for such a tiny thing :-)