cairoLineTo. Adds a line to the
path from the current point to a point that is offset from the
current point by (dx, dy) in user space. After this call the
current point will be offset by (dx, dy).cairoRelLineTo(cr, dx, dy)cr, dx, dy)
is logically equivalent to cairo_line_to (cr, x + dx, y + dy).
It is an error to call this function with no current point. Doing
so will cause cr to shutdown with a status of
CAIRO_STATUS_NO_CURRENT_POINT.