QEM decimator: vertex anchoring feature, to prevent them from being merged
Fixed QEM least error vertex computation
QEM decimator fix: orphan vertices no longer added to the final result

This patch kinda forced the FixInversion() function to be removed from the
interface, it is now called automatically, and thus the user must ensure
that the vertices pointer given to Set() is still valid when Process() is
called. The Get() routine also got some slight improvements.
Added some doc for SCE_MC_GenerateVertices()
Fixed anchoring in decimator module
QEM decimator: anchored vertices never ever get merged
Je suis un programmateur de Funès.
QEM decimator: avoid over-collapsing
QEM decimator: edge length is added to the error instead of multiplied