Option to give existing canvas element as winit window

[smokku]

This PR extends WindowDescriptor resource with canvas field.
This is a Document.querySelector() string allowing pointing winit to an existing HTMLCanvasElement to use.

This allows embedding Bevy game in an existing web page.

[cart]

Ooh does this mean we might be able to have multiple canvases that behave as different "bevy windows"?

[smokku]

We are able to do so already. Every Bevy/winit window creates a separate <canvas> HTML element.

This PR gives an option to use pre-existing canvas element instead of creating and attaching one.

[cart]

Looks good to me. Its worth noting that I don't see an error/panic like I would expect to when the custom canvas doesnt exist.

[smokku]

You need to configure it. If it is not configured, canvas element is is created and attached to HTML.

Works for me.

diff --git a/examples/wasm/index.html b/examples/wasm/index.html
index 33f18ca2..f10014d8 100644
--- a/examples/wasm/index.html
+++ b/examples/wasm/index.html
@@ -21,7 +21,7 @@
     </style>
   </head>
   <script type="module">
-    import init from './target/headless_wasm.js'
+    import init from './target/winit_wasm.js'
     init()
   </script>
 </html>
diff --git a/examples/wasm/winit_wasm.rs b/examples/wasm/winit_wasm.rs
index 7759d141..c3ab5568 100644
--- a/examples/wasm/winit_wasm.rs
+++ b/examples/wasm/winit_wasm.rs
@@ -16,6 +16,7 @@ fn main() {
         .add_resource(WindowDescriptor {
             width: 300,
             height: 300,
+            canvas: Some("#nonexistant".to_string()),
             ..Default::default()
         })
         .add_default_plugins()

gives: