backends: Move MetaOutput::crtc field into private struct

No functional changes. This is only done so that changes to reference
counting can done more reliably.

https://bugzilla.gnome.org/show_bug.cgi?id=786929

src/backends/meta-output.c

@@ -21,7 +21,13 @@
 
 #include "backends/meta-output.h"
 
-G_DEFINE_TYPE (MetaOutput, meta_output, G_TYPE_OBJECT)
+typedef struct _MetaOutputPrivate
+{
+  /* The CRTC driving this output, NULL if the output is not enabled */
+  MetaCrtc *crtc;
+} MetaOutputPrivate;
+
+G_DEFINE_TYPE_WITH_PRIVATE (MetaOutput, meta_output, G_TYPE_OBJECT)
 
 MetaGpu *
 meta_output_get_gpu (MetaOutput *output)
@@ -29,6 +35,33 @@ meta_output_get_gpu (MetaOutput *output)
   return output->gpu;
 }
 
+void
+meta_output_assign_crtc (MetaOutput *output,
+                         MetaCrtc   *crtc)
+{
+  MetaOutputPrivate *priv = meta_output_get_instance_private (output);
+
+  g_assert (crtc);
+
+  priv->crtc = crtc;
+}
+
+void
+meta_output_unassign_crtc (MetaOutput *output)
+{
+  MetaOutputPrivate *priv = meta_output_get_instance_private (output);
+
+  priv->crtc = NULL;
+}
+
+MetaCrtc *
+meta_output_get_assigned_crtc (MetaOutput *output)
+{
+  MetaOutputPrivate *priv = meta_output_get_instance_private (output);
+
+  return priv->crtc;
+}
+
 static void
 meta_output_finalize (GObject *object)
 {