[CMAKE] Add the background image to the dist directory

Summary:
This diff generates the background image to be added to the OSX dist
directory. This background image will later be used as the background
for the OSX installer (DMG image).

This background file is expected to be a multi-resolution TIFF image. In
order to allow to change the package name easily, it is constructed from
a SVG file template which is configured at build time.

This SVG file is then converted to multiple DPI raster images, which are
finally concatenated into a single final background TIFF file.

This image transformation process requires several steps, and will
differ between a cross compilation and a native compilation due to the
different tool being available. This diff only support cross
compilation, native platform support will be added later.

Depends on D4380.

Test Plan:

ninja osx-deploydir

Check the background.tiff file is added to the dist directory (under
a dedicated .background directory).
Open the TIFF file and check it contains 2 images with different (x2
ratio) resolutions.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4381