input-surface: Don't hide keyboard on small downswipes

When testing this after release I discovered the feature because the keyboard would disappear mid-word. Turns out I swipe slightly down while typing fast, resulting in this gesture being triggered.

Initially, using gtk_gesture_get_bounding_box was attempted, but this doesn't work on touch screens. Instead, the MIN_Y_VELOCITY below was determined by repeatedly triggering the issue and finding a velocity that is above those.