Generate baseline screenshots

[Cliabhach]

We ultimately want some sort of visual regression testing capability. It will compare known good screenshots against the current output of the codebase. In the ideal case, this could all be done with the BridgeContext and associated preview-mode code modules. Not sure. Look into libraries like PhantomCSS and scalpel, maybe whatever Dolphin is using, see what would work best on the JVM.

[Cliabhach]

To address that point about Dolphin: FIFO CI may be appropriate on a larger project, but this one is small and our images are more deterministic.

Anyway, I made a start on all this with dd5eac1, and work is continuing in the branch for issue 37