x/mosaic rendering width does not bake in assumptions about character aspect ratio

### Description

Given the example in x/mosaic, if I were instead to remove the lines enforcing a height and width, the documentation states that the default here should be the base image height and width. Given the input image of a square --

```golang
m := mosaic.New() // .Width(80).Height(40)
```

I would expect that the terminal render would still be a square. However, we see the actual output

The fix seems to be manually changing the render width to twice the image size, as reported by `img.Bounds().Max.X`, i.e.

```golang
mosaic.New().Width(img.Bounds().Max.X * 2).Render(...)
```

Is this behavior expected?

### Version

_No response_

### Environment

_No response_