do not serialize glyph advances, glyph offsets will reflect absolute glyph positions.
Since: 1.8.0. Note: when this flag is used with a partial range of the buffer (i.e. start is not 0), calculating the
absolute positions has a cost proportional to start. If the buffer is serialized in many small chunks, this can lead to
quadratic behavior. It is recommended to use a larger buf_size to minimize this cost.