fix: magick previewer for multi-layered image files with -flatten argument

[v3natio]

As the title describes it, this is an easy fix in order for the previewer to work for xcf and other files which handle layers. It should work as normal for non-layered files.

[sxyazi]

Could you please send me a file with layers for me to test?

[v3natio]

Alright, so for example for the following .xcf file (uploading it as zip since github won't let me otherwise):
test.zip

We get the following previews using the old command (no -flatten arg):

And we get a singular image using the arg:

[sxyazi]

Tested and works great, thanks for the fix! 👍🏻