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Fixes [#146251](https://github.com/flutter/flutter/issues/146251). On Windows, a "pointer add" event to a new view is sometimes sent before the "pointer remove" event from the old view. When `PointerDataPacketConverter` handles the add event, it asserts that the device's pointer state has been cleared. Since the pointer state is cleared only when the remove event is processed, the assertion fails if the events arrive out of order. The solution proposed here is to synthesize a remove event if an add event is received while the pointer state hasn't been cleared (i.e., if the pointer is added without being removed from the previous view). To avoid duplicate remove events, the view ID is now tracked in `PointerState`. If the original "remove" arrives after the synthesized one, the state's view ID will differ from the pointer data's view ID, allowing it to be safely ignored. When synthesizing the remove event, it's possible that the old view has already been destroyed, meaning no remove event will be received later. For example, this occurs when a window is destroyed while the pointer is inside it. However, the framework expects an "add" event to always follow a "remove" event, and "remove" events with invalid views are ignored. To meet the framework's expectations, the view ID of the "add" event will be used for the "remove" event if the old view has been destroyed. [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style