Support multiple WebGL contexts

Right now, kiss3d only supports rendering to a single `` element. I have a use case where I'd like to embed multiple instances of kiss3d on the same page (a blog post that goes through implementing a 3d simulation).

I've identified a few blockers:

* [WebGLCanvas always looks for a single canvas on the page](https://github.com/sebcrozet/kiss3d/blob/48985e838f4946aacf2db269909f8c5a95455f9c/src/window/webgl_canvas.rs#L46) (`#canvas`). This can be worked around on the JavaScript side by changing the id of the target canvas before creating the window, and then changing it back afterwards.
* [WebGLContext does the same thing](https://github.com/sebcrozet/kiss3d/blob/48985e838f4946aacf2db269909f8c5a95455f9c/src/context/webgl_context.rs#L27). Now this one is more complicated to work around because [kiss3d only ever keeps a single context in memory](https://github.com/sebcrozet/kiss3d/blob/48985e838f4946aacf2db269909f8c5a95455f9c/src/context/context.rs#L116). One solution to this is to instantiate one WASM module per canvas (+ the trick for supporting multiple canvases).
* Mouse and keyboard events are relayed to all canvases because [they all listen on window](https://github.com/sebcrozet/kiss3d/blob/48985e838f4946aacf2db269909f8c5a95455f9c/src/window/webgl_canvas.rs#L78). As a temporary fix on my fork, I've added the following check inside the event handlers:
```rust
if e.target()
.and_then(|target| ::try_from(target).ok())
.map_or(true, |canvas| canvas != edata.canvas)
{
return;
}
```
**EDIT:** For the last point, a better solution is to add event listeners to the canvas element directly (except for window resize / mouse up / mouse move (which needs to be modified to support the pointer leaving the canvas).