Rename hidpi_factor to scale_factor (#1334)

* Rename hidpi_factor to scale_factor

* Deprecate WINIT_HIDPI_FACTOR environment variable in favor of WINIT_X11_SCALE_FACTOR

* Rename HiDpiFactorChanged to DpiChanged and update docs

I'm renaming it to DpiChanged instead of ScaleFactorChanged, since I'd
like Winit to expose the raw DPI value at some point in the near future,
and DpiChanged is a more apt name for that purpose.

* Format

* Fix macos and ios again

* Fix bad macos rebase

src/platform_impl/web/event_loop/window_target.rs

@@ -185,7 +185,7 @@ impl<T> WindowTarget<T> {
                     height: raw.height() as u32,
                 };
 
-                backend::window_size().to_physical(backend::hidpi_factor())
+                backend::window_size().to_physical(backend::scale_factor())
             } else {
                 intended_size
             };