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/web.rs

@@ -3,7 +3,10 @@
 //! The web target does not automatically insert the canvas element object into the web page, to
 //! allow end users to determine how the page should be laid out. Use the `WindowExtStdweb` or
 //! `WindowExtWebSys` traits (depending on your web backend) to retrieve the canvas from the
-//! Window.
+//! Window. Alternatively, use the `WindowBuilderExtStdweb` or `WindowBuilderExtWebSys` to provide
+//! your own canvas.
+
+use crate::window::WindowBuilder;
 
 #[cfg(feature = "stdweb")]
 use stdweb::web::html_element::CanvasElement;
@@ -20,3 +23,31 @@ use web_sys::HtmlCanvasElement;
 pub trait WindowExtWebSys {
     fn canvas(&self) -> HtmlCanvasElement;
 }
+
+#[cfg(feature = "stdweb")]
+pub trait WindowBuilderExtStdweb {
+    fn with_canvas(self, canvas: Option<CanvasElement>) -> Self;
+}
+
+#[cfg(feature = "stdweb")]
+impl WindowBuilderExtStdweb for WindowBuilder {
+    fn with_canvas(mut self, canvas: Option<CanvasElement>) -> Self {
+        self.platform_specific.canvas = canvas;
+
+        self
+    }
+}
+
+#[cfg(feature = "web-sys")]
+pub trait WindowBuilderExtWebSys {
+    fn with_canvas(self, canvas: Option<HtmlCanvasElement>) -> Self;
+}
+
+#[cfg(feature = "web-sys")]
+impl WindowBuilderExtWebSys for WindowBuilder {
+    fn with_canvas(mut self, canvas: Option<HtmlCanvasElement>) -> Self {
+        self.platform_specific.canvas = canvas;
+
+        self
+    }
+}