Update ImageWorkshopLayer.php

Fixing a bug when adding a layer on top after resizing with converse proportions.

To recreate the bug:

```php
$watermark = PHPImageWorkshop\ImageWorkshop::initFromPath('watermark.png');
$watermark->opacity(20);

$image = PHPImageWorkshop\ImageWorkshop::initFromPath('image.png');
$image->resizeInPixel(770, 770, true, 0, 0, 'MM');
$image->addLayerOnTop($watermark);
```

Expected result:
image.png resized to 770 x 770px with the watermark on top with opacity 20%

Actual result:
blank image resized to 770 x 770px with the watermark on top with opacity 20%