Reduce WM_PAINT messages of thread target window

[wusyong]

Fixes #2367

Since there's no comment on the issue, I would like to open a PR and give it a try. Examples are working and we can still get nonstop messages with ControlFlow::Poll if we want.

• 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 or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[maroider]

I'll be sure to have a look at this once I feel I have a better grasp of what exactly this part of the code is supposed to do. I'm afraid to touch it with my current level of understanding.

[kchibisov]

Is this still an issue on the latest master?