Preserve transparency when pasting with alpha on the Imagick driver

[nlemoine]

paste() with an alpha < 100 on the Imagick driver paints a "black box" over the transparent areas of the pasted image. Paste a watermark that has a transparent surround at, say, 50% opacity and the whole bounding box comes back as a semi-opaque dark rectangle instead of just the visible part fading.

Culprit is setImageAlpha($alpha / 100) (and the setImageOpacity fallback). Both set every pixel's alpha to the same value, so fully-transparent pixels turn semi-opaque. GD doesn't have this problem because imagecopymerge blends without flattening the source alpha.

The fix scales the existing alpha instead of overwriting it: evaluateImage(EVALUATE_MULTIPLY, $alpha / 100, CHANNEL_ALPHA), after activating the alpha channel. Transparent stays transparent, opaque fades.

[mlocati]

👍

[nlemoine]

Thank you @mlocati!

[mlocati]

Thanks to you for these fixes and related tests!