Preserve a transparent base when pasting with alpha on the GD driver - #881

GD counterpart of #878, which fixed the same thing for Imagick. Found it while pasting a watermark over a transparent PNG.

Problem: Gd\Image::paste($image, $start, $alpha) sends 0 < alpha < 100 through imagecopymerge(). That function only reads the source RGB (it ignores the source per-pixel alpha), and every destination pixel it touches ends up opaque. So when the base has transparency, the area under the overlay is forced opaque and its color gets pulled towards whatever RGB the transparent base stored, which is white for a normal PNG. You end up with an opaque near-white block where the paste happened. An opaque base (a JPEG) is fine, it only breaks when the base itself has transparency.

Minimal repro:

use Imagine\Gd\Imagine;
use Imagine\Image\Box;
use Imagine\Image\Point;
use Imagine\Image\Palette\RGB;

$imagine = new Imagine();
$palette = new RGB();

$base = $imagine->create(new Box(20, 20), $palette->color([255, 255, 255], 0)); // transparent
$overlay = $imagine->create(new Box(10, 10), $palette->color([0, 128, 0]));      // opaque green
$base->paste($overlay, new Point(0, 0), 40);

$base->getColorAt(new Point(5, 5));
// before: #99cc99, alpha 100  (opaque, washed towards white)
// after:  #008000, alpha 40   (overlay color at 40%, base stays semi-transparent)

Fix: fade the overlay's own alpha channel by the opacity factor, then paste at full opacity so it goes through the alpha-aware imagecopy() path instead of imagecopymerge(). I clone the overlay first so the caller's image is left untouched. Same approach as #878 did for Imagick (evaluateImage(MULTIPLY, factor, CHANNEL_ALPHA) then composite), and as Intervention/image#1489 did for GD. GD has no native equivalent of Imagick's evaluateImage() on the alpha channel, so the overlay alpha is scaled pixel by pixel.

Opaque bases behave exactly as before, testPasteWithAlpha and testPasteWithAlphaPreservesTransparency still pass with the same values.

Testing: I added testPasteWithAlphaPreservesTransparentBase next to the existing opaque-base test, in AbstractImageTest so every driver runs it. It fails on GD before the change, passes on Imagick, and is skipped on Gmagick (no alpha paste there). Full suite is green in the 8.1-gd-imagick docker image (GD bundled 2.1 compatible), 783 tests, and I also ran it against a newer GD (2.3.3) locally.