I have finally managed to fix starling graphics extension drawing api and implement all 3 joint types (Miter, Bevel and Round). I still need to fix one edge case when angle between lines is extremely sharp but all 3 joint types work correctly now.
Now starling graphics extension supports line drawing that is almost identical as one in flash graphics api plus you can use textures as fill.
This was so painful to implement I was working on it for two weeks 😀 But now it works great.
Here is the lib:
And this is what I have implemented except for the arc joint type: