trim() function should only trim transparent pixels but trims black pixels too

[chescos]

What are the steps to reproduce?

Trim the following image:

You'll get this result:

What is the expected behaviour?

Only trim transparent pixels instead of transparent AND black pixels.

Are you able to provide a minimal, standalone code sample, without other dependencies, that demonstrates this problem?

const fs = require('fs');
const sharp = require('sharp');

const originalFileBuffer = fs.readFileSync('input.png');

(async () => {
    const newFileBuffer = await sharp(originalFileBuffer)
      .trim(1)
      .toFormat('png')
      .toBuffer();

    fs.writeFileSync('output.png', newFileBuffer);
})();

Run the above code snippet with this image as input.png:

What is the output of running npx envinfo --binaries --system?

  System:
    OS: Linux 5.11 Ubuntu 21.04 (Hirsute Hippo)
    CPU: (16) x64 AMD Ryzen 7 PRO 5850U with Radeon Graphics
    Memory: 18.05 GB / 30.26 GB
    Container: Yes
    Shell: 5.1.4 - /bin/bash
  Binaries:
    Node: 14.17.5 - /usr/bin/node
    npm: 6.14.14 - /usr/bin/npm

[lovell]

Please see #2166