Chunks fix

[wzman]

Since face Select variable is sU8 - chunks op no longer works.

DivideInChunks calculation use as trick the Select variable to temporaly store index, with a range to -1..X (signed 32 bits). This trick no longer works since its a sU8.

I suggest this pull to fix chunks operator (without many changes and works) :

• Add an extended Faces structure (with a sF32 Select variable)
• temporaly copy Faces into this extended structure and compute chunk on it.
• after calculation copy the result into Faces and clear unused extended structure.

[rygorous]

No, that hack with re-using Select to store the chunk index is totally horrible anyway, and recreating that situation again is a cure worse than the disease.

I'll fix this properly tomorrow...