[mutter] monitor-transform: Add generic function to get relative transform

[Jonas Ådahl <jadahl src gnome org>]

commit d600cd9aeef5206787c60e8ba93fd82e1f17a971
Author: Robert Mader <robert mader posteo de>
Date:   Mon Jun 24 21:26:31 2019 +0200

    monitor-transform: Add generic function to get relative transform
    
    It takes transforms A and B and returns the transform to get from
    A to B.
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/526

 src/backends/meta-monitor-transform.c | 27 +++++++++++++++++++++++++++
 src/backends/meta-monitor-transform.h |  3 +++
 2 files changed, 30 insertions(+)
---
diff --git a/src/backends/meta-monitor-transform.c b/src/backends/meta-monitor-transform.c
index e8672d151..f0d3fd52e 100644
--- a/src/backends/meta-monitor-transform.c
+++ b/src/backends/meta-monitor-transform.c
@@ -53,3 +53,30 @@ meta_monitor_transform_transform (MetaMonitorTransform transform,
 
   return new_transform;
 }
+
+/**
+ * meta_monitor_transform_relative_transform:
+ * @transform: The transform to start from
+ * @other: The transform to go to
+ *
+ * Return value: a transform to get from @transform to @other
+ */
+MetaMonitorTransform
+meta_monitor_transform_relative_transform (MetaMonitorTransform transform,
+                                           MetaMonitorTransform other)
+{
+  MetaMonitorTransform relative_transform;
+
+  relative_transform = ((other % META_MONITOR_TRANSFORM_FLIPPED -
+                         transform % META_MONITOR_TRANSFORM_FLIPPED) %
+                        META_MONITOR_TRANSFORM_FLIPPED);
+
+  if (meta_monitor_transform_is_flipped (transform) !=
+      meta_monitor_transform_is_flipped (other))
+    {
+      relative_transform = (meta_monitor_transform_invert (relative_transform) +
+                            META_MONITOR_TRANSFORM_FLIPPED);
+    }
+
+  return relative_transform;
+}
diff --git a/src/backends/meta-monitor-transform.h b/src/backends/meta-monitor-transform.h
index 86dc9692d..f73abe79a 100644
--- a/src/backends/meta-monitor-transform.h
+++ b/src/backends/meta-monitor-transform.h
@@ -58,4 +58,7 @@ META_EXPORT_TEST
 MetaMonitorTransform meta_monitor_transform_transform (MetaMonitorTransform transform,
                                                        MetaMonitorTransform other);
 
+MetaMonitorTransform meta_monitor_transform_relative_transform (MetaMonitorTransform transform,
+                                                                MetaMonitorTransform other);
+
 #endif /* META_MONITOR_TRANSFORM_H */