fix: Screenshot capture in non-main thread setting

[bitsandfoxes]

Resolves #2387

Unity does not permit the start of coroutines outside the main thread. When inside a burst job, this will actually cause a crash.

The changes here do not just prevent the crash but also enable the SDK to capture a screenshot when an event gets captured on a background thread.
This is achieved by checking whether the current thread is the main thread and if not, "enqueuing" the coroutine to be invoked on the next frame.

I'll follow up on the docs with the limitation of capturing the screenshot on the next frame. The fact that an event is not coming from the main thread is on the issue itself, see