How does sharp infer the height if the width is provided?

Nobody has claimed this yet.

## Question about an existing feature

### What are you trying to achieve?
I am writing a webpack loader to optimize images, some of that work includes resizing. For dev purposes I need to know up-front what will be the image dimensions. As the user can specify the width only I need to infer the height. After looking at sharp source code I found these lines:
https://github.com/lovell/sharp/blob/7c631c0787915416e20a567a039516e99c81c42d/src/pipeline.cc#L176-L184

I thought that this must be it so I reimplemented this logic in my loader:
```js
// https://github.com/lovell/sharp/blob/7c631c0787915416e20a567a039516e99c81c42d/src/pipeline.cc#L176-L184
function inferHeight(currentWidth, currentHeight, newWidth) {
const xFactor = currentWidth / newWidth;
const newHeightNotRounded = currentHeight / xFactor;
const newHeight = Math.round(newHeightNotRounded);
return newHeight;
}
```

Unfortunately there must be more to it. For the target width of `496` and current height of `2731` and current width of `4096` this code inferred `331`. While sharp resized the image to `330`. It's not the rounding semantics and it's not the numerical precision, I check these already.

So the question stands, how does sharp auto-scale the images?

Best,
Wiktor

### Please provide a minimal, standalone code sample, without other dependencies, that demonstrates this question

I've created a smal example with the image that behaves in this way (other images work with the inference code). Running index.js in this repo, will run sharp and this inference function: [The link to a reproduction repo](https://github.com/watjurk/sharp-resize-inference)

### Please provide sample image(s) that help explain this question

[The sample image](https://github.com/watjurk/sharp-resize-inference/blob/master/image.jpeg)

Start with src/pipeline.cc at lines 176-184 and run index.js from the linked reproduction repository against image.jpeg. Compare the loader's inferred dimensions with sharp's output for width 496, current dimensions 4096×2731, and trace the relevant resize behavior. Done means explaining why sharp produces height 330 while the shown calculation produces 331.

Written by the indexing model from the issue text.