[Forwardport] Fixes black background for png images in wysiwyg editors.

[nmalevanec]

Original Pull Request

#16733

Description

This PR fixes the black background for png images, which are uploaded in wysiwyg editors. This issue appears only for GD2 image processing library.

Fixed Issues (if relevant)

1. Transparent background becomes black for thumbnails of PNG into Wysiwyg editor... #14248: Transparent background becomes black for thumbnails of PNG into Wysiwyg editor

Manual testing scenarios

1. Upload a png image from any wysiwyg editor
2. The background while previewing of this image should be transparent.
   

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @nmalevanec. Thank you for your contribution
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[miguelbalparda]

We'll go with #17855, thanks!