image-copy: Do not panic on ImageCaptureSourceKind::Destroyed

[ids1024]

The way this was handled previously is incorrect. We should still handle creation of a capture session, just send stopped when it is created.

This can be tested by creating a capture source and session for a workspace that has been removed. Toplevel and output sources have a different issue in smithay: Smithay/smithay#1961

Should fix pop-os/cosmic-epoch#3319.

https://gist.github.com/ids1024/808358ed4671af7a8b31edc97a45cccc has a minimal test client using cosmic-client-toolkit to reproduce the issue; by trying to create a capture session for a workspace as soon as it is removed. Prior to this PR, that crashed the compositor when a workspace is removed. After it, the compositor runs fine, and that shows the capture session is stopped.

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[Drakulix]

Thanks!