Handle transparency (shadows) in images with composite layout

[MatthiasPortzel]

macOS and GNOME's screenshot tools (and probably others) can include a drop-shadow behind and around screenshots of windows. These screenshots look better, in my subjective opinion, and I would like to be able to use them in Catwalk'd screenshots in the Sublime Text port (for example).

However, right now the way that catwalk implements the composite layout stacks the four screenshots on top of each-other, leading to the semi-transparent shadow not being even in the resulting image (despite being uniform in the input screenshots).