Allow zooming with two fingers on touchpads and mobile devices

この issue には説明がありません。

The issue body names no files, tests, or entry points. Start by locating the existing zoom and pointer or touch input handling, then determine how two-finger gestures are represented on touchpads and mobile devices; done means two-finger movement zooms the canvas on both device types without breaking existing input.

索引モデルが issue の本文から書いたものです。