The first character of the ID type was used to tag IDs for updates which
is weak since different IDs can have the same first character (for
example meshes, materials and metaballs), causing unnecessary updates of
unrelated IDs.
Now we use a unique index per ID type to tag for updates, unifying IDs
arrays indexing along the way.
Reviewers: sergey, mont29
Differential Revision: https://developer.blender.org/D2139
b745a2401b