Blender Git Loki

Fix T77261: Multires creates spikes when subdividing ngons

The spikes were caused by non-initialized tangent matrix used during
smoothing process. The reason tangent matrix was not initialized was
because wrong usage of API: n-gons should pass corner of 0 to the
matrix construction function.

Corrected usage of the API and added assert() to help catching such
kind of non-initialized issues easier.