Add pinch zoom support

[bear24rw]

On macOS trackpad two finger vertical scroll zooms and two finger horizontal scroll pans. This leads to some funky user experience where you accidentally either zoom when trying to pan or pan when trying to zoom. It might be better to require holding shift or something to enable zoom and make vertical scroll pan by default? (or vice versa)

[jcupitt]

Hello @bear24rw.

I think the right fix would be to add support for pinch-zoom gestures. gtk4 has this, it probably just needs hooking up.

Let's tag this as an enhancement.