ImDrawList: add PathFillConcave(), AddConcavePolyFilled(): amends (#760)

- Simplify and compact some code. Shallow tweaks.
- Add comments.
- Add concave shape demo.
- Remove coarse culling.
- Remove nested types to match coding style and for consistent type nams when translated to other languages.
- Merged ClassifyNode() and ReclassifyNode().
- Extracted ImTriangleIsClockwise().
- Hold copy of points inside nodes instead of pointing to them.
5 files changed