Poly Line / Flattened Path API or Struct

[mokafolio]

Based on the discussion here https://github.com/mokafolio/Tarp/issues/8 it became clear that a way to create flattened linear segments directly through either a public facing struct or corresponding API functions will be useful to draw a lot of linear line segments and also might be a nice hook if you want to manually control how paths are being flattened. This issue keeps track of the progress on this feature.