OpenXR: complete custom composition layer support

Development Platform & Builds XR Development
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https://github.com/EpicGames/UnrealEngine/pull/15038/commits

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Complete the extension hooks introduced by https://github.com/EpicGames/UnrealEngine/commit/fa9d8f00acff8654f131bbed25d4142f50e3879d so plugin-defined layer shapes can enter the native swapchain path and carry composition layer structures not known by OpenXRHMD.​

- let extension plugins opt custom shapes into native layer creation on the game thread

- add fixed-size aligned inline storage for extension-defined composition layer structures

- validate custom structures against the OpenXR base-header layout at compile time

- route header access, next-chain updates, and xrEndFrame submission through GetHeader()

- document the fixed storage size and alignment as an ABI contract

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previous EPS ticket: [Content removed]

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Here is an example implementation on external plugin：

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I’m not sure if there’s a better solution available.

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Hi Seamus,

Although it isn’t 100% confirmed, we’re currently planning a 5.9 release for Unreal Engine, and I’ve put this PR into the task list for it. I’ll try and give it a look in the next couple weeks and get back to you if it needs adjustments, or otherwise go ahead w/ submitting it to Main.

Thanks!

Christopher Fiala

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