This patch is extracted from soc-2014-shapekey branch.
Basic idea is to only write a skey vertex if it actually differs from org mesh, instead of writing coordinates for all vertices of all shapekeys.
Benefits hugely to file size in some cases (heavy meshes with tens of shapes, like this one (a face with many expression keys limitted to an area, like lip, eyes, nose, etc.). With this mesh I get:
New format quite obviously breaks forward compatibility, exactly like BMesh did some times ago. So patch also adds a 'compat' flag to force writing skeys in 'old' format.
Now, aside from code itself (it’s quite straight-forward I think), main question is, do we accept to break file format for this? If so, do we do it now, or rather wait 2.8 to do it together with other forward-compat breaking changes?