drawLine() ignores transparent colors with GD <= 2.1.0

Describe the bug

The drawLine() function ignores a transparent color and draws the line completely opaque. #ffffff44 for example is drawn as #ffffffff.

The erroneous output was observed with GD version bundled (2.1.0 compatible), which is now about 8 years old and no longer supported. However, it still seems to be unfortunately present in many installations. For example, a recent official PHP 8.3 Docker image still contains this old version.

With a setup with the same version the line width is also ignored. See #1304

There were no problems with current GD versions (e.g. 2.3.3).

Code Example

$image = $manager
    ->create(500, 500)
    ->fill('ff5500')
    ->drawLine(function (LineFactory $line) {
        $line->color('fff4'); // transparency!
        $line->from(100, 100);
        $line->to(400, 100);
    });

Expected behavior

Function should draw line (half) transparent according to given color.

Environment

• PHP Version: 8.1
• OS: Debian
• Intervention Image Version: 3.6.3
• GD or Imagick: GD bundled (2.1.0 compatible)

Start by reproducing the drawLine() example with GD bundled 2.1.0-compatible, then trace the drawLine() entry point and its GD-specific handling. Done means transparent colors render with the requested alpha on the affected GD version while current GD behavior remains intact; the report does not name files or tests.