This makes it so coordinates and normals for CCG are calculated
with mutires displacement taken into account. This solves issues
with multires displacement being lost when entering sculpt mode.
The missing part is averaging of normals along grid boundaries.
But even then sculpting shows decent results.
The plan to solve that would be to introduce function to stitch
grids, which can also be used by Smooth brush which requires
this.
41c039d6e9