1 bit dithered PNG without palette

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## Question about an existing feature

### What are you trying to achieve?

I am trying to make an 1 bit per pixel PNG image, but I want the colors to be pure black and white. When setting colours to 2, it uses a palette that may contain grayscales, which looks really bad when tiled.

I tried manually setting only options.pngBitdepth, but then no dithering occurs.

### When you searched for similar issues, what did you find that might be related?

https://github.com/lovell/sharp/issues/2855
https://github.com/lovell/sharp/issues/2828

Some issues about getting dithered images, or 1 bit images, but everyone seems fine with a potentially gray palette.

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

extracted from https://github.com/maptiler/tileserver-gl/blob/master/src/serve_rendered.js but modified to output 1 bit dithered black and white.
```js
const image = sharp(data, {
raw: {
width: params.width * scale,
height: params.height * scale,
channels: 4
}
});
image.toColourspace('b-w');
image.removeAlpha();

image.png({ adaptiveFiltering: false, dither: 1.0, colours: 2});
image.toBuffer((err, buffer, info)
```

### Please provide sample image(s) that help explain this question
This is what the tiles look like with this option


The entry point is image.png({ adaptiveFiltering: false, dither: 1.0, colours: 2 }) in the supplied JavaScript sample; compare it with related issues #2855 and #2828. Done means producing a 1-bit dithered PNG whose palette contains only pure black and white.

Written by the indexing model from the issue text.