Implement hidpi for web platform (#1233)

* fix: use a 'static lifetime for the web backend's `Event` types

* implement hidpi for stdweb (web-sys wip?)

* fix: make all canvas resizes go through backend::set_canvas_size

* update Window docs for web, make `inner/outer_position` return the position in the viewport

src/platform_impl/web/stdweb/mod.rs

@@ -5,7 +5,7 @@ mod timeout;
 pub use self::canvas::Canvas;
 pub use self::timeout::Timeout;
 
-use crate::dpi::LogicalSize;
+use crate::dpi::{LogicalSize, Size};
 use crate::platform::web::WindowExtStdweb;
 use crate::window::Window;
 
@@ -41,6 +41,28 @@ pub fn window_size() -> LogicalSize<f64> {
     LogicalSize { width, height }
 }
 
+pub fn hidpi_factor() -> f64 {
+    let window = window();
+    window.device_pixel_ratio()
+}
+
+pub fn set_canvas_size(raw: &CanvasElement, size: Size) {
+    use stdweb::*;
+
+    let hidpi_factor = hidpi_factor();
+
+    let physical_size = size.to_physical::<u32>(hidpi_factor);
+    let logical_size = size.to_logical::<f64>(hidpi_factor);
+
+    raw.set_width(physical_size.width);
+    raw.set_height(physical_size.height);
+
+    js! {
+        @{raw.as_ref()}.style.width = @{logical_size.width} + "px";
+        @{raw.as_ref()}.style.height = @{logical_size.height} + "px";
+    }
+}
+
 pub fn is_fullscreen(canvas: &CanvasElement) -> bool {
     match document().fullscreen_element() {
         Some(elem) => {