#17 Make sure that the close_holes function does not create impossible shapes

[Frode]

Make sure that the close_holes function does not create impossible shapes / shapes that have both sides of each triangle defined as both inside and outside. This can probably happen when adding triangles in the middle of a hole, seamingly splitting a hole in two while what is really happening is that you still only have one hole if you try to follow the extent of the hole. The easiest way to fix this is probably to reorder the indices of each triangle such that it is known by looking at the index order what is inside and what is outside.