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

Cargo.toml

@@ -90,6 +90,7 @@ version = "0.3.22"
 optional = true
 features = [
     'console',
+    'CssStyleDeclaration',
     'BeforeUnloadEvent',
     'Document',
     'DomRect',