Support Mojave dynamic wallpapers

[sindresorhus]

Probably not going to be easy.

[sindresorhus]

By support, I mean being able to extract the currently shown image from the series of images it has. It already correctly sets and gets the special HEIC image used for dynamic wallpapers.

[sindresorhus]

Radar: https://openradar.appspot.com/radar?id=6086072953995264

[jshier]

This may be of use: https://nshipster.com/macos-dynamic-desktop/