Correct button state on synthetic pointer events

[cbracken]

Description

This corrects the button state emitted on synthetic pointer move and
hover events generated by the engine.

When an embedder notifies the engine of a pointer up or down event, and
the pointer's position has changed since the last move or hover event,
PointerDataPacketConverter generates a synthetic move or hover to
notify the framework of the change in position. In these cases, the
current event from the embedder contains the new button state after
the pointer up/down event, but the move/hover needs to be synthesized
such that it occurs before the pointer up/down, with the previous
button state.

This patch stores the button state after each pointer down, up, move, or
hover event such that it can be used by the next event if a synthetic
event must be issued.

The bug in the previous logic was revealed by the release of macOS 11
(Big Sur), which appears to issue move events between mouse down and
mouse up, which did not use to be the case.

Related Issues

This fixes flutter/flutter#64961, which is the
desktop-specific tracking bug for the more general Big Sur mouse click
umbrella issue flutter/flutter#71190.

Tests

I added the following tests:

• Test that synthesised move and hover events have the correct button state.

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• All existing and new tests are passing.
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Reviewer Checklist

• I have submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

Breaking Change

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[chunhtai]

Thanks for fixing this. This change looks good, just need a test

[chunhtai]

We may have same issue in web pointer converter https://github.com/flutter/engine/blob/master/lib/web_ui/lib/src/engine/pointer_converter.dart

Not sure if this will affect web

[cbracken]

The web side was already landed in #22813. Test WIP but should be momentarily :)

[cbracken]

Tests added -- caught an issue with synthesised events emitted when scrolling.

[chunhtai]

LGTM

[ferhatb]

LGTM