Blender Git Loki

Replace stupid static balancing with task-based one

Initially i wanted to have some really simple and basic
threading scheduler and wrote one based on traversing
depsgraph in advance. But it ended up in some issues with
the single-pass traverse i did which didn't gather all
the dependencies actually.

That was for sure solvable, but it ended up in a bit of
time consuming thing and with huge help of Brecht's
patch it was faster just to write proper balancing.

But it's again really basic thing, which could be
easily changed depending on feedback and design decisions
from Joshua,

So for now it works in the following way:

- Currently DagNode is used for threaded evaluaiton,
meaning traversing actually happens for DagNodes.

This is easier than converting DAG to a graph where
only objects are stored, but required adding one int
field to DagNode for faster runtime checks.

We could change this later when it'll be clear how
and where we'll store evaluation data, but for now
it work pretty ok.

- The new field is called "valency" and it's basically
number of node parents which needs to be evaluated
before the node itself could be evaluated.

- Nodes' valency is getting initialized before threading,
and when node finished to update valency of it's childs
is getting decreased by one. And if it happens so
child's valency became zero, it's adding to task pool.

- There's thread lock around valency update, it'll be
replaced with spinlock in nearest future.

- Another update runtime data is node color. White nodes
represents objects, gray one non-objects.

Currently it's needed to distinguish whether we need to
call object_handle_update on node->ob or not. In the
future it could be replaced with node->type to support
granularity, meaning we then could update object data
separately from object itself.

- Needed to add some public depsgraph functions to make
it possible to traverse depsgraph without including
depsgraph private header to other files.

This change doesn't make code anyhow more stable, but
solves update order issues noticed while working on
fixing underlying bugs.

Threaded update is still ifdef-ed for until curves and
armatures are considered thread-safe, which is next
step to be done.