Better placeholder texture (or remove it?)

[jabooth]

Right now we show a rather uninspiring blurry placeholder image when we start loading a new asset. It's not exactly attractive, and may not even be required any more (before we moved to the current promise-based system, I think we needed to have a texture available at all times, but I can't think why that would be the case now).

Maybe we just rip it out? We could also make the spinner our dark blue colour to fit in with the theme.

Not high priority at all, just a nice aesthetic improvement.