On Web, use the new WebCanvasWindowHandle

[daxpedda]

Now we don't have to set data-raw-handle on the canvas anymore.

[daxpedda]

Unfortunately WebCanvasWindowHandle is supposed to be !Send, for good reasons. Currently asking in IRC how other platforms are doing this.

[daxpedda]

So I went ahead and made Window::window_handle() return HandleError::Unavailable when not on the main thread. To do this I had to change the call to raw_window_handle_rwh_06() from Window because it was using maybe_wait_on_main().

@madsmtm I didn't actually change anything in MacOS/iOS behavior because it still calls maybe_wait_on_main() with the SendSyncWrapper there, but maybe you want to change that to return HandleError::Unavailable as well?

[madsmtm]

maybe you want to change that to return HandleError::Unavailable as well?

Sure, see #3288.

[daxpedda]

Alright, let me go ahead and merge this then.

[madsmtm]

But returning WebCanvasWindowHandle instead of WebWindowHandle, while it shouldn't be a breaking change, in practice it will be, since a lot of users will assume that it returns WebWindowHandle, and panic if it doesn't.

So I think you should update the changelog to reflect that.

[daxpedda]

But returning WebCanvasWindowHandle instead of WebWindowHandle, while it shouldn't be a breaking change, in practice it will be, since a lot of users will assume that it returns WebWindowHandle, and panic if it doesn't.

Hm, alright.