Fix imagick stale layers

[nlemoine]

Resize an animated GIF on the Imagick driver, then ask for its layers: you get the pre-resize frames back.

$image = $imagine->open('animated.gif'); // 2 frames, 8x8
$image->resize(new Box(4, 4));

$image->getSize()->getWidth();                                // 4
$image->layers()->coalesce()->get(0)->getSize()->getWidth();  // 8, expected 4

Layers grabs the \Imagick instance in its constructor, and Image::layers() caches the object forever. That was harmless until the multi-frame paths started swapping the instance: coalesceImages()/deconstructImages() return a new \Imagick, which resize(), crop(), flatten() and the optimize save path assign to $this->imagick, so the cached Layers keeps reading frames nothing else uses anymore. One concrete casualty: animated.delay/animated.loops are applied through layers() in prepareOutput(), land on the orphaned frames, and the saved file keeps its original timing.

Reassignments now go through a small setter that drops the cached Layers when the instance changes (__clone() already rebuilds it the same way), so the next layers() call reads the live resource.

[mlocati]

Nice catch!