Windows: Increase border size in undecorated shadow mode for better grip

[sout233]

After enabling undecorated shadow on Windows, the borders, except for the top, become very thin, making dragging the window difficult.

I have modified the border size to make it easier to grab and drag. However, I've realized that this might not be the best solution.

• [ ] Tested on all platforms changed
• [x] 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
• [ ] Created or updated an example program if it would help users understand this functionality