Mac trackpad gestures

[moffatman]

Start sending PointerPanZoom events from macOS when trackpad is used

Fixes flutter/flutter#23604

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[chinmaygarde]

@cbracken Ping?

[justinmc]

I tried this on my Macbook along with the framework PR (flutter/flutter#89944) using my demo app and it worked very well 👍 .

Would it be possible to write a unit test for this in FlutterViewControllerTest.mm?

[zanderso]

From PR review triage: It looks like the next step on this one is for @moffatman to respond to the feedback.

[chinmaygarde]

Ping @justinmc.

[chinmaygarde]

@dkwingsmt I believe this is good for another review.

[justinmc]

LGTM 👍 But @dkwingsmt should verify that the tests look ok.

Mostly just a bunch of nitpick comments on missing periods from me.

[dkwingsmt]

LGTM except for the periods.

[cbracken]

This looks great with one tiny nit.