Fix: Preserve signature in PNG palette images during transformation

[clementbirkle]

Before this change, when transforming PNG palette images (with transparency), the signature would be lost, and the resulting image would become fully transparent. This PR ensures that the signature is preserved during transformations, keeping it visible while maintaining the transparency of the image.

Screenshots:

• Original Image (before transformation):
  
• Image Before Fix (signature lost after transformation):
  
• Image After Fix (signature preserved):
  

[freekmurze]

Thank you! It seems that the tests are failing. Could you take a look?

[clementbirkle]

Thank you for bringing this up!

All tests pass locally with my Lando config:

name: spatieimage
recipe: lamp
config:
  webroot: .
  php: "8.2"
services:
  appserver:
    build_as_root:
      - apt-get update

      # Install NodeJS
      - curl -sL https://deb.nodesource.com/setup_20.x | bash -
      - apt-get install -y nodejs

      # Install dependencies needed for optimizers and PHP extensions
      - apt-get install -y jpegoptim pngquant gifsicle optipng libjpeg-progs webp libavif-bin

It seems that my local environment is not generating the same snapshots as required by GitHub Actions.
@freekmurze, could you help me investigate this? I haven’t been able to pinpoint the issue.