Fix Windows fullscreen toggle on windows that started invisible

[repi]

If one created a window with with_visibility(false) and then done show() then set_fullscreen wasn't working properly and hid the window because the internal WindowFlags was still stored as WindowFlags::VISIBLE = false.

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[Osspial]

Thanks for the PR! Could you rebase this onto the eventloop-2.0 new master branch?

[Osspial]

I think this has already been fixed on master, actually.

[repi]

Thanks, we haven't upgraded and tested 0.20 yet but great if it has been fixed there with/after the big refactor. Can confirm later when we upgrade

[Osspial]

It looks like this particular issue has been addressed, so I'm going to close this PR. However, it looks like the test-case you describe is still causing some unrelated funky behavior which I'll look into.

Looks like I was wrong about this being fixed on master :V! The issue I was running into was fixed by the changes included in this patch, so I've updated them for master and re-submitted them as #1345, so this should be addressed in the next release.

[repi]

Thanks!