Blender Git Loki

Fix inconsistent creation of `NodeTreeTypeUndefined` type of node tree

Currently, when creating a new node tree ID, its `typeinfo` is set to
`NodeTreeTypeUndefined`, but its `type` enum value is left to `0`,
aka `NTREE_SHADER`.

This patch adds a new `NTREE_UNDIFINED` value, and use it for
`NodeTreeTypeUndefined` types of node trees.

NOTE: While it is not clear whether that actually fixes issues currently,
quite a bit of code still relies on the value of `type`, so think it
makes sense to sanitize this.

NOTE: Would have been ideal to reserve `0` value to undefined type,
but at this point this is not possible anymore, so chose to use `-2` instead.

Reviewed By: JacquesLucke

Differential Revision: https://developer.blender.org/D13123