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Multires: Unsubdivide and Rebuild Subdivisions

This implements the main unsubdivide algorithm which rebuilds a base mesh and extracts the grid's data from a high resolution mesh.
It includes the Rebuild Subdivisions operator, which generates all subdivision levels down to the level 0 base mesh.

It supports:
- Rebuilding an arbitrary number of levels (Unsubdivide) or as many levels as possible down to level 0 in a single step (Rebuild Subdivisions).
- Rebuilding with already existing grids.
- Meshes with n-gons and triangles
- Meshes with more than 2 faces per edge
- Base mesh made completely out of triangles
- Meshes without poles
- Meshes with multiple disconnected elements at the same subdivision level

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D7372