Face orientation

[Eneroth3]

Currently face orientation matters to SolidOperations. When intersection edges have been formed in both meshes corresponding are found. Sometimes these should be removed, e.g. when two volumes that just touches are united (opposite orientation). Sometimes these should be kept (and merged into one face), e.g. overlapping volumes being united.

As of now the operation fails or produces unexpected results when faces are reversed.

The solid operations would feel more stable and friendly if face orientation didn't matter.

However, it is not acceptable to simply orient all faces at the start of the operation, as this will make the resulting geometry have face orientation that corresponds to the input geometry. To the end user a face can seemingly change material, when in fact the desired material was applied to the back face, now unexpectedly being "correctly" oriented.