Put move_skybox into CoreStage::PostUpdate

[TGRCdev]

I first used this plugin in combination with the bevy_flycam plugin, but when i wrote a simple test program, occasionally the skybox would rapidly jitter whenever the camera moved.

skybox.mp4

I fixed this by creating a stage label SkyboxStage, which is a single system stage that runs after CoreStage::Update and only contains the move_skybox system. (This PR now puts move_skybox into CoreStage::PostUpdate instead of its own stage.) My rationale is that any method that moves the camera will most likely be within the Update stage, and this change will improve bevy_skybox as a drop-in plugin without any camera system needing to be aware of its ordering.

I initially tried to fix this by going into bevy_flycam and adding .before("skybox") to the systems that moved the camera, but this output the error "system wants to run before unknown label 'skybox'" and did not fix the issue. Thus, I believe that this is the correct fix to this problem.

[alice-i-cecile]

Moving this plugins' systems to CoreStage::PostUpdate will achieve a similar effect of "correct behavior by default" and be less intrusive.

More generally, this plugin should be exporting a nice typed system label or two to allow users to control within stage ordering.

Thanks for bringing this up, this is a great simple and concrete example of the problems that bevyengine/rfcs#33 tries to address.

[TGRCdev]

My bad! I assumed that CoreStage::PostUpdate would execute after the frame rendered, but this assumption was wrong. Changed and squashed.

[TGRCdev]

Also added two typed system labels, SkyboxInitSystemsLabel for the startup systems and SkyboxSystemLabel for move_skybox

This change should be made in a separate PR

[jomala]

Thanks for the contribution, @TGRCdev !