macOS: Feature-gate CGSSetWindowBackgroundBlurRadius

[madsmtm]

Feature-gate CGSSetWindowBackgroundBlurRadius behind the Cargo feature private-apple-apis.

Fixes #4205.

• Tested on all platforms changed
  • We don't really have an example to test it properly (since all our examples render something static), but calling set_blur(true) works both with and without the feature enabled.
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior

[peterklingelhofer]

Hit this on the exhale MAS submission this week. The feature-gate approach in this PR matches what we ended up shipping as a local vendor/winit patch.

[danifunker]

Is there any update on this PR? Would be nice to not have to write a custom patch every time I need to submit a new app for MAS to review :)

[pszsh]

I just saw this thread after I submitted my PR.

#4607

I'm somewhat eager to hear what y'all think. I think it might make everyone happy. If not, I thought of one other way to do it besides this way. So, let me know.

[tronical]

I'll try to shepherd this change in. #4632 seems like a sensible follow-up @pszsh that IMO should be rebased once this is in.