[gtk/wip/chergert/macos-iosurface] macos: add comment about purging IOSurface

[Christian Hergert <chergert src gnome org>]

commit 0fbce5260e6f10e724ae01920a568dd950a63b7b
Author: Christian Hergert <christian hergert me>
Date:   Sat Feb 12 02:32:20 2022 -0800

    macos: add comment about purging IOSurface

 gdk/macos/gdkmacosbuffer.c | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/gdk/macos/gdkmacosbuffer.c b/gdk/macos/gdkmacosbuffer.c
index 2982d39594..804572d313 100644
--- a/gdk/macos/gdkmacosbuffer.c
+++ b/gdk/macos/gdkmacosbuffer.c
@@ -52,6 +52,13 @@ gdk_macos_buffer_dispose (GObject *object)
     g_critical ("Attempt to dispose %s while lock is held",
                 G_OBJECT_TYPE_NAME (self));
 
+  /* We could potentially force the unload of our surface here with
+   * IOSurfaceSetPurgeable (self->surface, kIOSurfacePurgeableEmpty, NULL)
+   * but that would cause it to empty when the layers may still be attached
+   * to it. Better to just let it get GC'd by the system after they have
+   * moved on to a new buffer.
+   */
+
   g_clear_pointer (&self->surface, CFRelease);
   g_clear_pointer (&self->damage, cairo_region_destroy);