touch_TouchscreenZoom uses two gesture files per device.

With all zooms, do not make stray gestures.  Use only two fingers.  Once the
zoom starts, do not stop movement or lift fingers until gesture is complete.

<DEVICE_NAME>_touchscreen_zoom_in:
    Press two fingers to the touchscreen such that they are close together, but
    not touching.  Smoothly spread them apart.

<DEVICE_NAME>_touchscreen_zoom_out:
    Press two fingers to the touchscreen such that they are separated.  Smoothly
    bring them together, but do not let them touch.