The open source OpenXR runtime

doc: Document !2490

Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2490>

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doc/changes/compositor/mr.2490.md
··· 1 + main: Add debug image target, lets you build and run the main compositor without 2 + any windowing backend. Images could be viewed in debug UI or read back using 3 + the u_var system.