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.

docs/CHANGELOG.txt

@@ -47,6 +47,8 @@ Other changes:
   frames would erroneously close the window. While it is technically a popup issue
   it would generally manifest when fast moving the mouse bottom to top in a sub-menu.
   (#7325, #7287, #7063)
+- DrawList: Added AddConcavePolyFilled(), PathFillConcave() concave filling. (#760) [@thedmd]
+  Note that only simple polygons (no self-intersections, no holes) are supported.
 - Docs: added more wiki links to headers of imgui.h/imgui.cpp to facilitate discovery
   of interesting resources, because github doesn't allow Wiki to be crawled by search engines.
   - This is the main wiki: https://github.com/ocornut/imgui/wiki