[mutter] monitor-config-manager: reuse old CRTC when possible

[Jonas Ådahl <jadahl src gnome org>]

commit f4f823f2380b94e97f7d7952bae200ab7e791699
Author: Emilio Pozuelo Monfort <emilio pozuelo collabora co uk>
Date:   Tue Nov 6 15:03:50 2018 +0100

    monitor-config-manager: reuse old CRTC when possible
    
    We shouldn't change an output's CRTC if we don't have to, as
    that causes the output to go black.
    
    This patch depends on
    "monitor-unit-tests: initial crtcs in custom_lid_switch".
    
    https://gitlab.gnome.org/GNOME/mutter/issues/373

 src/backends/meta-monitor-config-manager.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)
---
diff --git a/src/backends/meta-monitor-config-manager.c b/src/backends/meta-monitor-config-manager.c
index 431f1c1c3..335c3f34b 100644
--- a/src/backends/meta-monitor-config-manager.c
+++ b/src/backends/meta-monitor-config-manager.c
@@ -2,6 +2,7 @@
 
 /*
  * Copyright (C) 2016 Red Hat
+ * Copyright (c) 2018 DisplayLink (UK) Ltd.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -96,11 +97,16 @@ static MetaCrtc *
 find_unassigned_crtc (MetaOutput *output,
                       GPtrArray  *crtc_infos)
 {
+  MetaCrtc *crtc;
   unsigned int i;
 
+  crtc = meta_output_get_assigned_crtc (output);
+  if (crtc && !is_crtc_assigned (crtc, crtc_infos))
+    return crtc;
+
   for (i = 0; i < output->n_possible_crtcs; i++)
     {
-      MetaCrtc *crtc = output->possible_crtcs[i];
+      crtc = output->possible_crtcs[i];
 
       if (is_crtc_assigned (crtc, crtc_infos))
         continue;