Set up testing with `nv-flip-rs`

[zmitchell]

The wgpu crate uses https://github.com/gfx-rs/nv-flip-rs to test some of its rendering. To have better testing infrastructure we should:

• Create snapshots of all example outputs that aren't animations
• Run testing in CI against the snapshots

[zmitchell]

This is probably also related to #50