win32: avoid overflow when negating wheel deltas

[mikamikasuki]

Fixes #4722.

Both horizontal Win32 wheel paths negate a signed i16 before converting it to f32. For i16::MIN, that overflows in debug builds and wraps in release builds. Convert to f32 before negation and share the conversion helper.

Add a regression test for i16::MIN and the ordinary ±120 wheel deltas, and note the fix in the changelog.

Validation:

• cargo +1.98.1 check -p winit-win32 --tests --target x86_64-pc-windows-gnu
• cargo +nightly fmt --all -- --check
• git diff --check

The Windows-only test cannot run on this macOS host; Windows CI can execute it.

• Tested on all platforms changed
• Added an entry to the changelog module
• Updated documentation beyond the changelog
• Created or updated an example program