[CMAKE] Build the background dist image on native OSX

Summary:
D4381 introduced the generation of the background image to the dist
directory. This diff adds support for the image transformation using
tools available on OSX.

Depends on D4381.

Test Plan:
On OSX:

ninja osx-deploydir

Check the background file is correctly generated.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4382