Overwrite opaque pixels with transparent ones when compositing

Nobody has claimed this yet.

## Question about an existing feature

### What are you trying to achieve?
I want to place an image with transparency on top of another, but cut through the original image so it is still fully transparent.
I looked through the blend modes and I couldn't figure out how to do this without creating another image to use as a mask.

### When you searched for similar issues, what did you find that might be related?
This was somewhat related, but I couldn't find any cutout method in the current version: https://github.com/lovell/sharp/issues/1016

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

```js
const a = await sharp("A.png").toBuffer();
const b = await sharp("B.png").toBuffer();

const c = await sharp(b)
.composite([{ input: a, left: 0, top: 0 }])
.toFile("C.png");
```
### Please provide sample image(s) that help explain this question

A:
B:
A and B combined (I want the center to be transparent like this):

Start with the standalone JavaScript example using composite and review the related discussion in issue #1016. Determine whether the existing compositing behavior can meet the requested cutout result or whether a new capability is needed; done means the center becomes transparent without requiring a separate mask image.

Written by the indexing model from the issue text.