Adds support for pointer signals, in a way that will support both discrete events (e.g., scroll wheels, flutter/flutter#22762) and continuous gestures (e.g., trackpad scroll, flutter/flutter#21953).
Also exposes these new event options to the embedder. Does not include code to send the
new events from the platform shells.
Rather than using mojom to encode pointer data, we now encode and decode it
manually. A future patch will remove the mojom codepath once the framework is
updated.
