White or Black bars with PNG and transparent background

[troyere]

Q	A
Bug report?	no
Feature request?	yes
Usage question?	yes
PHP version used	7.2

Hello,

I have a similar problem of the following issue : #95

In short, black bars are appearing in my case too.

I searched in the library code, and I found this :
https://github.com/Sybio/ImageWorkshop/blob/2.0.9/src/PHPImageWorkshop/Core/ImageWorkshopLayer.php#L1564

Why don't we just add imagecolortransparent($image) in the next line, in case of $backgroundColor equals transparent ? I tested it directly in the lib, it works like a charm ^^'

Thanks in advance for your answer :)