Fix duplicated pointer axis events

In pointer.c, some axis event was emitted even if the event pointer did not have
current axis.

In X11 backend pointer scroll events seem to be composed of both BUTTON_PRESS
and BUTTON_RELEASE. Therefore we should skip one of them (RELEASE) to avoid
event duplication.

backend/libinput/pointer.c

@@ -128,7 +128,7 @@ void handle_pointer_axis(struct libinput_event *event,
 			}
 			wlr_event.delta = libinput_event_pointer_get_axis_value(
 					pevent, axies[i]);
-		}
 			wl_signal_emit(&wlr_dev->pointer->events.axis, &wlr_event);
 		}
 	}
+}

backend/x11/backend.c

@@ -63,8 +63,7 @@ static bool handle_x11_event(struct wlr_x11_backend *x11, xcb_generic_event_t *e
 		x11->time = ev->time;
 		break;
 	}
-	case XCB_BUTTON_PRESS:
+	case XCB_BUTTON_PRESS: {
-	case XCB_BUTTON_RELEASE: {
 		xcb_button_press_event_t *ev = (xcb_button_press_event_t *)event;
 
 		if (ev->detail == XCB_BUTTON_INDEX_4 ||
@@ -79,7 +78,16 @@ static bool handle_x11_event(struct wlr_x11_backend *x11, xcb_generic_event_t *e
 				.delta = delta,
 			};
 			wl_signal_emit(&x11->pointer.events.axis, &axis);
-		} else {
+			x11->time = ev->time;
+			break;
+		}
+	}
+	/* fallthrough */
+	case XCB_BUTTON_RELEASE: {
+		xcb_button_press_event_t *ev = (xcb_button_press_event_t *)event;
+
+		if (ev->detail != XCB_BUTTON_INDEX_4 &&
+				ev->detail != XCB_BUTTON_INDEX_5) {
 			struct wlr_event_pointer_button button = {
 				.device = &x11->pointer_dev,
 				.time_sec = ev->time / 1000,