MeshDecimate: optimize checking for normal inversion

[Fedr]

In MeshDecimator::canCollapse_

1. Store new directed areas of only triangles that flip their normal or became not-degenerate from degenerate (and not all triangles as previously).
2. If at least one remaining triangle flips its normal, checks that new normal is consistent with the average normal of new vertex neighborhood (and do not check triangles that did not flip their normals).

Overall it gives about 5% speed up in decimation.