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/event.rs

@@ -292,21 +292,21 @@ pub enum WindowEvent<'a> {
     /// Touch event has been received
     Touch(Touch),
 
-    /// The DPI factor of the window has changed.
+    /// The window's scale factor has changed.
     ///
     /// The following user actions can cause DPI changes:
     ///
     /// * Changing the display's resolution.
-    /// * Changing the display's DPI factor (e.g. in Control Panel on Windows).
-    /// * Moving the window to a display with a different DPI factor.
+    /// * Changing the display's scale factor (e.g. in Control Panel on Windows).
+    /// * Moving the window to a display with a different scale factor.
     ///
     /// After this event callback has been processed, the window will be resized to whatever value
     /// is pointed to by the `new_inner_size` reference. By default, this will contain the size suggested
     /// by the OS, but it can be changed to any value.
     ///
     /// For more information about DPI in general, see the [`dpi`](dpi/index.html) module.
-    HiDpiFactorChanged {
-        hidpi_factor: f64,
+    DpiChanged {
+        scale_factor: f64,
         new_inner_size: &'a mut PhysicalSize<u32>,
     },
 
@@ -394,7 +394,7 @@ impl<'a> WindowEvent<'a> {
             }),
             Touch(touch) => Some(Touch(touch)),
             ThemeChanged(theme) => Some(ThemeChanged(theme)),
-            HiDpiFactorChanged { .. } => None,
+            DpiChanged { .. } => None,
         }
     }
 }