feature: Add an into() bevy mesh, example viewer.

[shanecelis]

Add a cargo feature "into_bevy_mesh" that will provide a From implementation so generators like Cube can be turned into a bevy Mesh. This doesn't add any dependencies on the default features. It does add dependencies on bevy_render and glam when the feature is enabled.

Add an example called "viewer" that uses this feature to view the generators. It's a small bevy app with some camera control. I did discover a bug in a cone normal so plus one for visualization.

[shanecelis]

Here are some screenshots of the viewer and the cone with and without the normal fix.

[kvark]

Thank you for the PR!
I think this functionality is useful, but it should probably not live in genmesh itself. Problem is genmesh depending on bevy. This problem is two-fold:

1. Bevy should be a consumer of genmesh, and thus depend on it, not the other way around
2. Bevy version becomes a part of genmesh public API. E.g. once Bevy-0.11 is released, this code in genmesh becomes dead weight, and only pushes genmesh to publish another breaking release, which we'd want as little as possible.

[shanecelis]

Thanks for looking it over. I'll consider adding a PR to bevy with a "genmesh" feature. It's really the orphan rules that force this kind of feature to live in genmesh or bevy, but I agree bevy would be the right place for it to live.