Blender Git Loki

Depsgraph: Initial support for skipping invisible IDs update

General idea is quite simple here:

- Store layers in the IDDepsNode (same as we did in the old dependency graph).

This layer would correspond to ID layers themselves plus layers of nodes which
depends on it.

Here's where first tricky part begins: i'm not sure how we could flush layers
between nodes. Basically because it's totally unclear how do we perform full
DFS nowadays. Previously it was simple because of having root node and knowing
for sure nodes which doesn't have dependencies. Would need to think a bit how
to do it now.

- We keep track of currently visible layers.

Here's where tricky parts continues. basically the question is: where do we
store currently visible layers? Evaluation context comes in mind, but that'd
be wrong idea. basically because evaluation context is shared between scenes
and since it's possible to have two different scenes visible we can not store
single value.

So for now we store visible layers in the depsgraph itself. This way we're
decoupling visible layers between scenes. This is temporary solution for
until we'll have some more centralized storage. Should be easy to move stuff
around anyway, so wouldn't be so much fussed about this.

Another tricky part here is the render engines. Currently there's no visibility
check performed for render engine update.

So after all it seems we'll need to store visible layers somewhere and pass
what layers we're currently interested in to the depsgraph evaluation, making
it so we can easily use the same graph for viewport and render engine and
perform all the visibility checks we want.

- We don't schedule invisible IDs during evaluation.

We instead put them to the "to be updated when becoming visible list". Now,
this list is a bit of an overkill, would be better to have some sort of legacy
(ID->flag & ID_NEEDS_UPDATE) flag, but since we're currently not really sure
what the storage for such a flags would look like we'll keep a temp list for
now. Again, it'll be easy to change in the future, so wouldn't be fussed here
as well.

- When visibility changes depsgraph updates visible layers and re-schedules
updates of previously invisible objects.

TODOs:

- Implement layers visibility flush.
- Make sure set scenes works all nice.
- Move temp storage to somewhat final storage (well, implement that final
storage first :)