Switch to Bevy 0.8 for vertex colour support

[philpax]

Bevy 0.8 introduced vertex colour support in bevyengine/bevy#4528. I'd quite like to use it for rendering.

Doing this requires porting over bevy_egui. There's a fork from 30 days ago (at the time of writing) that does some of the work, but there have been more breaking changes. I'll have to get that fork working first.