custom skybox

[EricPedley]

follow-up to #2
Should be possible because of this PR to gz sim: gazebosim/sdformat#1037

[EricPedley]

Also, clouds aren't rendering :(
https://answers.gazebosim.org/question/20177/clouds-from-screen-is-not-visible-in-camera-image/
gazebosim/gazebo-classic#1757

[EricPedley]

Fixed by setting far plane to 10000 instead of leaving it out. It must've been getting set to a default value (should've made a separate issue for cloud rendering but whatever :/)

[EricPedley]

Ok it's actually just broken. gazebosim/gz-rendering#663