This avoids pointless allocations

These will only trigger the gesture if it's been created with
the same GtkGesture::n-points than n_fingers in the event.

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.

For now, at least. We do this by hiding the instance and
class structures in private headers.

It is generally more useful to get offsets right away,
than having to calculate those from the start point.

This gesture interprets and reports drags as an offset to the drag
start point.