Get a file descriptor for manual event polling

Right now the only way to get the events is to call the function `xrPollEvent`. It would be quite handy to be able to retrieve a pollable file descriptor (FD for short) from OpenXR and integrate it into another event loop, such as [wayland](https://wayland.freedesktop.org/docs/html/apc.html#Server-structwl__event__source_1a0e7ce1f52dfe04c73b6a7c2263c7ef25) or simply [epoll](https://man7.org/linux/man-pages/man7/epoll.7.html).

I would be interested to work on such extension, if it is deemed useful.

Start by reviewing the existing OpenXR event API around xrPollEvent and the requirements for integrating with Wayland or epoll. Define the proposed extension and its platform behavior; done means a documented, accepted design for retrieving a pollable file descriptor and using it for manual event polling.

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