[gtk/wip/chergert/quartz4u: 105/116] macos: add opacity helper

[Christian Hergert <chergert src gnome org>]

commit 4bca7b9a6be70b2664137d49b27fa75a33e01d97
Author: Christian Hergert <chergert redhat com>
Date:   Thu Jun 18 16:42:37 2020 -0700

    macos: add opacity helper

 gdk/macos/gdkmacossurface-private.h |  2 ++
 gdk/macos/gdkmacossurface.c         | 10 ++++++++++
 2 files changed, 12 insertions(+)
---
diff --git a/gdk/macos/gdkmacossurface-private.h b/gdk/macos/gdkmacossurface-private.h
index 217813dda5..f5a52db3cb 100644
--- a/gdk/macos/gdkmacossurface-private.h
+++ b/gdk/macos/gdkmacossurface-private.h
@@ -120,6 +120,8 @@ gboolean           _gdk_macos_surface_is_tracking             (GdkMacosSurface
 void               _gdk_macos_surface_monitor_changed         (GdkMacosSurface    *self);
 GdkMonitor        *_gdk_macos_surface_get_best_monitor        (GdkMacosSurface    *self);
 void               _gdk_macos_surface_reposition_children     (GdkMacosSurface    *self);
+void               _gdk_macos_surface_set_opacity             (GdkMacosSurface    *self,
+                                                               double              opacity);
 void               _gdk_macos_surface_get_root_coords         (GdkMacosSurface    *self,
                                                                int                *x,
                                                                int                *y);
diff --git a/gdk/macos/gdkmacossurface.c b/gdk/macos/gdkmacossurface.c
index 4e3d223044..aa5d99a095 100644
--- a/gdk/macos/gdkmacossurface.c
+++ b/gdk/macos/gdkmacossurface.c
@@ -1011,6 +1011,16 @@ _gdk_macos_surface_get_view (GdkMacosSurface *self)
   return [self->window contentView];
 }
 
+void
+_gdk_macos_surface_set_opacity (GdkMacosSurface *self,
+                                double           opacity)
+{
+  g_return_if_fail (GDK_IS_MACOS_SURFACE (self));
+
+  if (self->window != NULL)
+    [self->window setAlphaValue:opacity];
+}
+
 void
 _gdk_macos_surface_get_root_coords (GdkMacosSurface *self,
                                     int             *x,