SetWindowLongPtr on 32-bit

[This is an issue about my recent PR 384]

According to the documentation for SetWindowLongPtr,

When compiling for 32-bit Windows, SetWindowLongPtr is defined as a call to the SetWindowLong function.

What this really means is that calls to PInvoke.User32.SetWindowLongPtr on 32-bit processes will (and do) result in an EntryPointNotFoundException

Thoughts on how to, or whether to, fix this?

I feel like this should be fixed, by exposing SetWindowLongPtr through User32.Helpers.cs and conditionally calling SetWindowLongPtr on 64-bit vs. SetWindowLong on 32-bit.

If no objections, I'll submit another PR.

[AArnott]

Thanks for catching that. Your proposal for a helper method sounds reasonable. The only alternative would be to dual-compile the library for x86 and x64 and I'd rather avoid doing that.

I have a pull request out for this.