Blender Git Loki

Cryptomatte: Data structure in compositor node

This changes the way how the mattes are stored in the compositor node. This used to
be a single string what was decoded/encoded when needed. The new data structure
stores all entries in `CryptomatteEntry` and is converted to the old `matte_id`
property on the fly.

This is done for some future changes in the workflow where a more structured
approach leads to less confusing and easier to read code.