Attempts to substitude CCGDM with an OpenSubdiv based structure which has less abstraction levels. The missing part in this substitude is a face pointers which old CCGDM/multires code was using to stitch faces (averaging boundaries). Another curial bit missing: "reshaping" of multires CD_MDISPS to the state of new PBVH grids. The new code is only available when OpenSubdiv modifier is enabled (WITH_OPENSUBDIV_MODIFIER=ON) and with debug value of 128. This is so this WIP code is not interfering with current production machines in the studio. Reviewers: brecht Reviewed By: brecht Differential Revision: https://developer.blender.org/D3685