MacOS provides the NSEventTypeMagnify which is very similar to the
Gtk ZOOM gesture and NSEventTypeRotate which is very similar to the
Gtk Rotate gesture. Those two event sequences are translated to a
sequence of GDK_TOUCHPAD_PINCH events. This sequence is then detected
in the upper gtk layers as Gtk Zoom/Rotate Gestures.
https://bugzilla.gnome.org/show_bug.cgi?id=760276