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

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -698,8 +698,10 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
         col.enabled = ob.mode != 'EDIT'
         col.operator("object.multires_subdivide", text="Subdivide")
         col.operator("object.multires_higher_levels_delete", text="Delete Higher")
+        col.operator("object.multires_unsubdivide", text="Unsubdivide")
         col.operator("object.multires_reshape", text="Reshape")
         col.operator("object.multires_base_apply", text="Apply Base")
+        col.operator("object.multires_rebuild_subdiv", text="Rebuild Subdivisions")
         col.prop(md, "uv_smooth", text="")
         col.prop(md, "show_only_control_edges")
         col.prop(md, "use_creases")