Add the ability to pass a prebuilt canvas (#1394)

This allows Winit to take control of existing canvas elements in the
DOM, which is useful for web applications with other content in the
page.

src/platform_impl/web/window.rs

@@ -23,13 +23,13 @@ impl Window {
     pub fn new<T>(
         target: &EventLoopWindowTarget<T>,
         attr: WindowAttributes,
-        _: PlatformSpecificBuilderAttributes,
+        platform_attr: PlatformSpecificBuilderAttributes,
     ) -> Result<Self, RootOE> {
         let runner = target.runner.clone();
 
         let id = target.generate_id();
 
-        let mut canvas = backend::Canvas::create()?;
+        let mut canvas = backend::Canvas::create(platform_attr)?;
 
         let register_redraw_request = Box::new(move || runner.request_redraw(RootWI(id)));
 
@@ -281,5 +281,7 @@ impl Id {
     }
 }
 
-#[derive(Debug, Default, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct PlatformSpecificBuilderAttributes;
+#[derive(Default, Clone)]
+pub struct PlatformSpecificBuilderAttributes {
+    pub(crate) canvas: Option<backend::RawCanvasType>,
+}