examples/glupload: Update glutin to 0.26 with winit 0.24

Supersedes !409 (closed). Consider this draft until I have fully checked the changes made there - somehow missed that MR even though it's not hidden between many MRs at all...

Winit 0.19 uses uninitialized variables which is invalid since Rust 1.48, leading to a runtime panic 1. Updating to the latest version resolves these issues but requires significant refactoring since the event loop now runs entirely within a closure.

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And some more typo fixes and improvements.