GDK-Win32: Enable HiDPI support for Windows

This enables HiDPI support for GTK+ on Windows, so that the
fonts and window look better on HiDPI displays.  Notes for the current
work:

-The DPI awareness enabling can be disabled if and only if an application
 manifest is not embedded in the app to enable DPI awareness AND a user
 compatibility setting is not set to limit DPI awareness for the app, via
 the envvar GDK_WIN32_DISABLE_HIDPI.  The app manifest/user setting for
 DPI awareness will always win against the envvar, and so the HiDPI items
 will be always setup in such scenarios, unless DPI awareness is disabled.

-Both automatic detection for the scaling factor and setting the scale
 factor using the GDK_SCALE envvar are supported, where the envvar takes
 precedence, which will therefore disable automatic scaling when
 resolution changes.

-I am unable to test the wintab items because I don't have such devices
 around.

https://bugzilla.gnome.org/show_bug.cgi?id=768081

gdk/win32/gdkwindow-win32.h

@@ -96,6 +96,14 @@ enum _GdkW32WindowDragOp
 
 typedef enum _GdkW32WindowDragOp GdkW32WindowDragOp;
 
+typedef enum _GdkWin32MonitorDpiType
+{
+  MDT_EFFECTIVE_DPI  = 0,
+  MDT_ANGULAR_DPI    = 1,
+  MDT_RAW_DPI        = 2,
+  MDT_DEFAULT        = MDT_EFFECTIVE_DPI
+} GdkWin32MonitorDpiType;
+
 struct _GdkW32DragMoveResizeContext
 {
   /* The window that is being moved/resized */
@@ -333,6 +341,11 @@ struct _GdkWindowImplWin32
    * They are removed at the first opportunity (usually WM_INITMENU).
    */
   LONG_PTR temp_styles;
+
+  /* scale of window on HiDPI */
+  gint window_scale;
+  gint unscaled_width;
+  gint unscaled_height;
 };
 
 struct _GdkWindowImplWin32Class
@@ -352,6 +365,8 @@ void  _gdk_win32_window_tmp_reset_parent_bg (GdkWindow *window);
 
 void  _gdk_win32_window_update_style_bits   (GdkWindow *window);
 
+gint  _gdk_win32_window_get_scale_factor    (GdkWindow *window);
+
 G_END_DECLS
 
 #endif /* __GDK_WINDOW_WIN32_H__ */