[mutter] backends/native: Scale relative input motions with monitor scale

[Carlos Garnacho <carlosg src gnome org>]

commit e60dfd5b2325b87f76468b4f3a29baef4c94e273
Author: Jonas Ådahl <jadahl gmail com>
Date:   Fri Feb 3 13:12:01 2017 +0800

    backends/native: Scale relative input motions with monitor scale
    
    To allow for more natural pointer movements from relative pointer
    devices (e.g. mouse, touchpad, tablet tool in relative mode, etc), scale
    the relative motion from libinput with the scale of the monitor. In
    effect, this means that the pointer movement is twice as fast (physical
    movement vs numbers of pixels passed) as before, but it also means that
    the same physical movement crosses the distance in a GUI no matter if
    it is on a HiDPI monitor or not.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=778119

 src/backends/native/meta-backend-native.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)
---
diff --git a/src/backends/native/meta-backend-native.c b/src/backends/native/meta-backend-native.c
index 3cd1d11..90ab1e2 100644
--- a/src/backends/native/meta-backend-native.c
+++ b/src/backends/native/meta-backend-native.c
@@ -255,6 +255,25 @@ pointer_constrain_callback (ClutterInputDevice *device,
   constrain_all_screen_monitors (device, monitor_manager, new_x, new_y);
 }
 
+static void
+relative_motion_filter (ClutterInputDevice *device,
+                        float               x,
+                        float               y,
+                        float              *dx,
+                        float              *dy,
+                        gpointer            user_data)
+{
+  MetaMonitorManager *monitor_manager = user_data;
+
+  logical_monitor = meta_monitor_manager_get_logical_monitor_at (monitor_manager,
+                                                                 x, y);
+  if (!logical_monitor)
+    return;
+
+  *dx *= logical_monitor->scale;
+  *dy *= logical_monitor->scale;
+}
+
 static ClutterBackend *
 meta_backend_native_create_clutter_backend (MetaBackend *backend)
 {
@@ -270,6 +289,8 @@ meta_backend_native_post_init (MetaBackend *backend)
 
   clutter_evdev_set_pointer_constrain_callback (manager, pointer_constrain_callback,
                                                 NULL, NULL);
+  clutter_evdev_set_relative_motion_filter (manager, relative_motion_filter,
+                                            meta_backend_get_monitor_manager (backend));
 }
 
 static MetaIdleMonitor *