Add web fullscreen support (#1142)

Adds fullscreen using native web APIs to the stdweb and web-sys backends.

Due to limitations of browser APIs, requests for fullscreen can only be fulfilled during a short-lived user-triggered event. This commit does automatically handle that under the hood, but it does introduce unavoidable latency in full-screening the canvas.

src/platform_impl/web/stdweb/mod.rs

@@ -5,19 +5,24 @@ mod timeout;
 pub use self::canvas::Canvas;
 pub use self::timeout::Timeout;
 
+use crate::dpi::LogicalSize;
 use crate::platform::web::WindowExtStdweb;
 use crate::window::Window;
 
 use stdweb::js;
 use stdweb::web::event::BeforeUnloadEvent;
-use stdweb::web::html_element::CanvasElement;
 use stdweb::web::window;
 use stdweb::web::IEventTarget;
+use stdweb::web::{document, html_element::CanvasElement, Element};
 
 pub fn throw(msg: &str) {
     js! { throw @{msg} }
 }
 
+pub fn exit_fullscreen() {
+    document().exit_fullscreen();
+}
+
 pub fn on_unload(mut handler: impl FnMut() + 'static) {
     window().add_event_listener(move |_: BeforeUnloadEvent| handler());
 }
@@ -27,3 +32,21 @@ impl WindowExtStdweb for Window {
         self.window.canvas().raw().clone()
     }
 }
+
+pub fn window_size() -> LogicalSize {
+    let window = window();
+    let width = window.inner_width() as f64;
+    let height = window.inner_height() as f64;
+
+    LogicalSize { width, height }
+}
+
+pub fn is_fullscreen(canvas: &CanvasElement) -> bool {
+    match document().fullscreen_element() {
+        Some(elem) => {
+            let raw: Element = canvas.clone().into();
+            raw == elem
+        }
+        None => false,
+    }
+}