Fix HINSTANCE returned by raw_window_handle on 64 bit Windows

[madsmtm]

• Tested on all platforms changed
  • Tested with x86_64-pc-windows-gnu, but can't get my build system to produce a 32bit version, so can't test that :/
• Compilation warnings were addressed
• cargo fmt has been run on this branch
• cargo doc builds successfully
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
  • I didn't add it to the changelog since this is a very small fix.
• 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

We previously retrieved the value using GetWindowLongW, but the documentation explicity says that GetWindowLongPtrW should be used for pointers and handles (which is what we're fetching).

[msiglreith]

Thanks!