Auto merge of #23574 - asajeffrey:glutin-reduce-borrow-duration, r=paulrouget

Don't process events while borrowing the event loop

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At the moment, the glutin embedder holds onto a borrow of the events loop while processing events, which is dangerous if any of the event handlers end up using the events loop reentrantly. (This caused a panic while integrating https://github.com/servo/rust-webvr/pull/84.)

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because this is low-level embedding code

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