Feature Request: Unify background colors between render engines

[hybridherbst]

Currently all render engines come with different background colors. That makes comparing colors between renderers harder, especially for transparencies.

Would be great if all render engines could use a similar/the same background color by default.

[emackey]

That's probably a relatively simple change. Contributions welcome! 😄

For stable testing of translucency, I recommend placing an opaque background wall with a visible texture on it, for example such as AlphaBlendModeTest.