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/stdweb/canvas.rs

@@ -2,7 +2,7 @@ use super::event;
 use crate::dpi::{LogicalPosition, PhysicalPosition, PhysicalSize};
 use crate::error::OsError as RootOE;
 use crate::event::{ModifiersState, MouseButton, MouseScrollDelta, ScanCode, VirtualKeyCode};
-use crate::platform_impl::OsError;
+use crate::platform_impl::{OsError, PlatformSpecificWindowBuilderAttributes};
 
 use std::cell::RefCell;
 use std::rc::Rc;
@@ -43,12 +43,15 @@ impl Drop for Canvas {
 }
 
 impl Canvas {
-    pub fn create() -> Result<Self, RootOE> {
-        let canvas: CanvasElement = document()
-            .create_element("canvas")
-            .map_err(|_| os_error!(OsError("Failed to create canvas element".to_owned())))?
-            .try_into()
-            .map_err(|_| os_error!(OsError("Failed to create canvas element".to_owned())))?;
+    pub fn create(attr: PlatformSpecificWindowBuilderAttributes) -> Result<Self, RootOE> {
+        let canvas = match attr.canvas {
+            Some(canvas) => canvas,
+            None => document()
+                .create_element("canvas")
+                .map_err(|_| os_error!(OsError("Failed to create canvas element".to_owned())))?
+                .try_into()
+                .map_err(|_| os_error!(OsError("Failed to create canvas element".to_owned())))?,
+        };
 
         // A tabindex is needed in order to capture local keyboard events.
         // A "0" value means that the element should be focusable in

src/platform_impl/web/stdweb/mod.rs

@@ -72,3 +72,5 @@ pub fn is_fullscreen(canvas: &CanvasElement) -> bool {
         None => false,
     }
 }
+
+pub type RawCanvasType = CanvasElement;