Decimate Improvements for Flat Surfaces

Campbell did it again!BMesh: decimate improvement for flat surfacesPreviously decimate on flat areas of a mesh would more or less randomly collapse edges.
(giving bad topology).This commit includes a topology 'cost', so smaller edges on flat surfaces collapse first.The main reason behind this was for designing jewelry. Watch more example images and read the discussion on developer.blender.org. Quite interesting! Link: Commit logThanks a lot Campbell and everybody involved in testing and giving feedback.