Add basic gradient sky dome

[voxelias]

Mesh generator:

dotrix::Mesh::hemisphere("Hemisphere", 64, 32);

Skydome entity:

let skydome_entity = (SkyDome::default(),);
world
    .spawn(Some(skydome_entity))
    .last()
    .expect("Sky dome must be spawned");

Rendering:

let skydome_renderer = dotrix::skydome::RenderSkyDome::setup().create(display);
...
scheduler.add_task(skydome_renderer);