Blender Git Loki

Fix: Cryptomatte Metadata Trimmed to 1024.

When reading metadata from image files the metadata is trimmed to 1024.
For cryptomatte the metadata can contain json data and should not be
trimmed. Resulting additional checks in the manifest parser for
incomplete json data.

You could argue to add an exception for cryptomatte, but that would
still allows misuse. When the direction of this patch is accepted we
should consider removing `maxlen` from `IDP_AssignString` as it
doesn't seem to be used anywhere else.

Reviewed By: #images_movies, mont29, sergey

Differential Revision: https://developer.blender.org/D10825