This adds a new function which has as its only effect the elimination
of
the current point. This makes it much easier to use the various
cairo_arc calls when the initial line_to is not actually desired.
This function also unifies and generalizes the long-existing behavior
of cairo_line_to being treated as cairo_move_to when there is no
current point. With the addition of cairo_new_sub_path this becomes a
documented feature with similar behavior in cairo_curve_to as well.
