Blender Git Loki

Proper volume data storage in Cycles when syncing from Blender data.

Previously a viewport render would continuously keep adding Volume instances
to the VolumeManager data, because there was no way to detect existing volume
data. Now the `id_map` model known from meshes and other data types is used for
volume data as well.

Note that the VolumeKey currently simply uses an Object ID pointer, thus
assuming a single Volume per object. In the future volumes could become a
real ID type in Blender, or the specific volume could be narrowed down with
additional info in the key.