Now trackpad gestures will count as pointerCount=2 instead of 1. It makes it easier for people who want to have different behaviour for single-finger drag vs two-finger pan/zoom. Also fixed up `scale_test.dart` to verify `pointerCount` in more places.

Related: https://github.com/flutter/flutter/issues/13102
Fixes https://github.com/flutter/flutter/issues/140730