Diff UI: screenshot diff

[oliviertassinari]

As a user, I can compare two screenshots using one of the following methods.

Motivation

Helps better catching regression.

Detail

• [ ] superposition:
• [ ] onion skin:
• [ ] swipe:
• [ ] flicker: https://github.com/Galooshi/happo/issues/170#issue-196468932

[gregberge]

@oliviertassinari I think it's a feature, to start superposition is good.

[gregberge]

359 will address that.