gtk_gesture_drag_get_start_point and gtk_gesture_drag_get_offset
have out args that need to be annotated.
This commit adds the (out) and (nullable) annotations as appropriate.
https://bugzilla.gnome.org/show_bug.cgi?id=734292
This prevents some of our generic object implementation tests
from working with gesture objects. Instead, add g_return_if_fail
checks in all the gesture constructors.