size (width, height) is inconsistent

Even if one specifies a size of `100` the returned qr code has a width/height of 128. This is something related to the rust qrcode library. Perhaps the dimensions are predetermined to fit all of the QR squares.

To reproduce:

```elixir
q = Qrusty.qr!("https://elixir-lang.org/", :jpg, size: 100, ec: :m)
File.write("/test.jpg", q)
# see that test.jpg is NOT 100x100 pixels
```
