Windows are apparently effectively !Send on Windows and macOS, and the lifetime issues that Android's window handles have make marking window handles Send perhaps a bit suspect.

I believe we should treat RawWindowHandles somewhat like pointers, in that they can be constructed by safe code, but can't be used directly without unsafe. Thus, we should make the handles Send, but provide enough information in order for user code to tell if they're using the handle on the correct thread. Additionally, any API creating ready-made RawWindowHandles must be unsafe, as is already the case with HasRawWindowHandle. This all assumes we can find a workable solution to #84.