wait for render app when main world is dropped

[hymm]

Objective

• Try not to drop the render world on the render thread, and drop the main world after the render world.
• The render world has a drop check that will panic if it is dropped off the main thread.

Solution

• Keep track of where the render world is and wait for it to come back when the channel resource is dropped.

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Changelog

• Wait for the render world when the main world is dropped.

Migration Guide

• If you were using the pipelined rendering channels, MainToRenderAppSender and RenderToMainAppReceiver, they have been combined into the single resource RenderAppChannels.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[alice-i-cecile]

Good comments, and I like the constructor. Once the migration guide is ready I'll press the button.

[hymm]

Added the migration guide. Kept it pretty brief as the usage is pretty niche.