wayland: Filter scroll events based on source

The POINTER_EMULATED flag was a convenience to filter either
side of smooth/discrete events that we should ignore based on
the source.

This distinction was challenged, first by v120 mice that use
Clutter smooth events to deliver semi-discrete changes, second
by commit e0c4b2b241 ("backends/native: Mark the emulated smooth
scroll event as such") which made the smooth events be flagged
as emulated, and the discrete whole-step events marked as
real.

This distinction feels convenient for the time being, since
upper layers might be confused by real smooth scroll events
without finish flags. Adapt to this change at MetaWaylandPointer
so that we drop the POINTER_EMULATED check, and the events are
perhaps filtered based on their source and the preferred
wl_seat version of the client that we are talking to.

This handles the whole grid of combinations:
- wheel sources with wl_seat >=8 result in wl_pointer.axis_value120
  from "emulated" smooth scroll events, with value120 information.
- wheel sources with wl_seat < 8 result in wl_pointer.axis_discrete
  from "real" discrete scroll events.
- finger/continuous sources prefer smooth events. Previously, always
  non-emulated for those.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3642>

src/wayland/meta-wayland-pointer.c

@@ -706,6 +706,34 @@ handle_button_event (MetaWaylandPointer *pointer,
     }
 }
 
+static gboolean
+maybe_filter_scroll_event (const ClutterEvent *event,
+                           int                 client_version)
+{
+  ClutterScrollSource source;
+
+  source = clutter_event_get_scroll_source (event);
+
+  switch (clutter_event_get_scroll_direction (event))
+    {
+    case CLUTTER_SCROLL_UP:
+    case CLUTTER_SCROLL_DOWN:
+    case CLUTTER_SCROLL_LEFT:
+    case CLUTTER_SCROLL_RIGHT:
+      if (source == CLUTTER_SCROLL_SOURCE_WHEEL)
+        return client_version >= WL_POINTER_AXIS_VALUE120_SINCE_VERSION;
+
+      return TRUE;
+    case CLUTTER_SCROLL_SMOOTH:
+      if (source == CLUTTER_SCROLL_SOURCE_WHEEL)
+        return client_version < WL_POINTER_AXIS_VALUE120_SINCE_VERSION;
+
+      return FALSE;
+    }
+
+  return TRUE;
+}
+
 static void
 handle_scroll_event (MetaWaylandPointer *pointer,
                      const ClutterEvent *event)
@@ -718,9 +746,6 @@ handle_scroll_event (MetaWaylandPointer *pointer,
   MetaWaylandPointerClient *client;
   ClutterScrollFinishFlags finish_flags;
 
-  if (clutter_event_get_flags (event) & CLUTTER_EVENT_FLAG_POINTER_EMULATED)
-    return;
-
   client = pointer->focus_client;
   if (!client)
     return;
@@ -796,6 +821,9 @@ handle_scroll_event (MetaWaylandPointer *pointer,
     {
       int client_version = wl_resource_get_version (resource);
 
+      if (maybe_filter_scroll_event (event, client_version))
+        continue;
+
       if (client_version >= WL_POINTER_AXIS_SOURCE_SINCE_VERSION)
         wl_pointer_send_axis_source (resource, source);