Validate face buffer of subdivision meshes

[stefanatwork]

The half edge of a face is looked up through the prefix sum of the face valences stored in the face buffer, but this buffer was never validated.

A face with zero vertices produced a start offset that pointed behind the half edge array, thus dereferencing the half edge of such a face during commit read out of bounds. A face buffer whose valences sum up to more than the size of an index buffer caused the half edge array, which is sized through the index buffer, to get written out of bounds. The sum was further accumulated in 32 bit, thus it could also wrap around.

The face buffer now gets validated before the half edge structures are built, and inconsistent face and index buffers raise an RTC_ERROR_INVALID_OPERATION error. The check is performed before any state is updated, thus a rejected geometry is left unmodified.

[stefanatwork]

I ditched the excessive AI slop and went for a minimally invasive fix:
A face with zero edges at the end of a subd mesh would get starting index of the size of the halfEdge array, after running the parallel prefix sum. Later on, the code tried to check the validity of that half edge, but didn't check if it was out of bounds. My fix is now to skip looking at half edges when the face has zero edges, and to clamp the last index coming out of the prefix sum.