fix: GDHandler::save() removes transparency

[kenjis]

Description
Fixes #4178

How to Test:
Prepare sample.png with transparency.

<?php
namespace App\Controllers;
class Home extends BaseController
{
    public function index()
    {
        \Config\Services::image('gd')
            ->withFile(WRITEPATH . 'sample.png')
            ->reorient()
            ->save(WRITEPATH . 'new.png', 70);
    }
}

Run the controller, and see new.png.

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide