pointer: transform low-res to high-res axis events

Currently, the "wlr_event_pointer_axis" event stores low-resolution
values in its "delta_discrete" field. Low-resolution values are always
multiples of one, i.e., 1 for one wheel detent, 2 for two wheel
detents, etc.

In order to simplify internal handling of events, always transform in
the backend from the low-resolution value into the high-resolution
value.

The transformation is performed by multiplying by 120. The 120 magic
number is used by the kernel and it is exposed to clients in the
"WLR_POINTER_AXIS_DISCRETE_STEP" constant.

backend/libinput/pointer.c

@@ -116,6 +116,7 @@ void handle_pointer_axis(struct libinput_event *event,
 				libinput_event_pointer_get_axis_value(pevent, axes[i]);
 			wlr_event.delta_discrete =
 				libinput_event_pointer_get_axis_value_discrete(pevent, axes[i]);
+			wlr_event.delta_discrete *= WLR_POINTER_AXIS_DISCRETE_STEP;
 			wlr_signal_emit_safe(&pointer->events.axis, &wlr_event);
 		}
 	}

backend/wayland/pointer.c

@@ -184,7 +184,7 @@ static void pointer_handle_axis_discrete(void *data,
 		return;
 	}
 
-	pointer->axis_discrete = discrete;
+	pointer->axis_discrete = discrete * WLR_POINTER_AXIS_DISCRETE_STEP;
 }
 
 static const struct wl_pointer_listener pointer_listener = {

include/wlr/types/wlr_pointer.h

@@ -78,6 +78,8 @@ enum wlr_axis_orientation {
 	WLR_AXIS_ORIENTATION_HORIZONTAL,
 };
 
+#define WLR_POINTER_AXIS_DISCRETE_STEP 120
+
 struct wlr_pointer_axis_event {
 	struct wlr_pointer *pointer;
 	uint32_t time_msec;

types/seat/wlr_seat_pointer.c

@@ -301,7 +301,7 @@ void wlr_seat_pointer_send_axis(struct wlr_seat *wlr_seat, uint32_t time,
 			if (value_discrete &&
 					version >= WL_POINTER_AXIS_DISCRETE_SINCE_VERSION) {
 				wl_pointer_send_axis_discrete(resource, orientation,
-					value_discrete);
+					value_discrete / WLR_POINTER_AXIS_DISCRETE_STEP);
 			}
 
 			wl_pointer_send_axis(resource, time, orientation,