Add support for single-threaded environments

[ecton]

If we want to allow WASM applications to be built with appit, we need to have a code path that supports:

• Preventing opening more than one window
• Does not spawn any threads

[daxpedda]

A main use-case in Winit is integrating your Rust application into a bigger web page, which is the reason why Winit supports multiple windows in Wasm.

Personally I'm not a fan of that and I think if users want to put their Rust application into a bigger web page they should just put it into an IFrame or something. Though some users also want support of JS integration with their Winit app, again, not a fan.

Just putting it out there, personally I don't have a use-case for it nor do I favor it's support.

[ecton]

I hadn't even thought about that! For Gooey, I've been trying to decide if I would even use this. To me, this is more about supporting a game written in Kludgine, where you want to take over the whole browser window or a single canvas with the Kludgine app. I'm not sure I care about multi-canvas "app"s written in Kludgine -- I don't even want to think of the API to support that :)

For Gooey's Canvas, for a v1, I'm probably going to just wrap the browser's drawing APIs. Later, I'd love to use Kludgine, but I'm pretty sure it'll be using Kludgine without winit/appit -- just using wgpu to attach to the canvas directly.