UI/snapshot testing

Some parts of Imagine Engine are a bit tricky to fully test with unit tests. While we are getting to a pretty high coverage now, parts of the rendering code are not really covered by tests, making refactors & fixes slow and risky.

It would be interesting to experiment with UI testing or snapshot testing for some of these cases, where we could - for instance - render an object and assert that the visual outcome matched our expectation.

I think if we can combine this with our current suite of unit tests we will have a quite strong test suite and will be able to move faster with great quality 👍