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/event_loop/mod.rs

@@ -37,7 +37,8 @@ impl<T> EventLoop<T> {
 
     pub fn run<F>(self, mut event_handler: F) -> !
     where
-        F: 'static + FnMut(Event<T>, &root::EventLoopWindowTarget<T>, &mut root::ControlFlow),
+        F: 'static
+            + FnMut(Event<'static, T>, &root::EventLoopWindowTarget<T>, &mut root::ControlFlow),
     {
         let target = root::EventLoopWindowTarget {
             p: self.elw.p.clone(),