Always call this function after FT_Stroker_GetBorderCounts to get sure that there is enough room in your FT_Outline object to receive all new data.
When an outline, or a sub-path, is `closed', the stroker generates two independent `border' outlines, named `left' and `right'
When the outline, or a sub-path, is `opened', the stroker merges the `border' outlines with caps. The `left' border receives all points, while the `right' border becomes empty.
Use the function FT_Stroker_Export instead if you want to retrieve all borders at once.