Ignore unrelated cursor buttons while doing seat operations

* Click and hold a scrollbar
* Drag the cursor onto another surface
* While still holding the original button, press and release another
cursor button
* Things get weird

There's two ways to fix this. Either cancel the seat operation and do
the other click, or continue the seat operation and ignore the other
click. I opted for the latter (ignoring the click) because it's easier
to implement, and I suspect a second click during a seat operation is
probably unintentional anyway.

sway/input/cursor.c

@@ -754,11 +754,12 @@ void dispatch_cursor_button(struct sway_cursor *cursor,
 	}
 	struct sway_seat *seat = cursor->seat;
 
-	// Handle ending seat operation
+	// Handle existing seat operation
-	if (cursor->seat->operation != OP_NONE &&
+	if (cursor->seat->operation != OP_NONE) {
-			button == cursor->seat->op_button && state == WLR_BUTTON_RELEASED) {
+		if (button == cursor->seat->op_button && state == WLR_BUTTON_RELEASED) {
 			seat_end_mouse_operation(seat);
 			seat_pointer_notify_button(seat, time_msec, button, state);
+		}
 		return;
 	}