Blender Git Loki

Blender 2.8: Initial hook of layer collections evaluation in DEG

This moves selectability/visibility flag flush from some hardcoded
places in the code to depsgraph. This way it is possible to simply
tag depsgraph to update those flags and rest it'll do on it's own.

Using depsgraph for such flush is an overkill: those flags are fully
static and can not be animated, so it doesn't really make sense to
hook only those to depsgraph.

However, in the future we will have overrides on collections, which
ideally would need to be animatable and drivable and easiest way
to support this is to do this on depsgraph level, so it ensures
proper order of evaluation for animation and drivers. And it seems
logical to do both overrides and flags flush from depsgraph from
this point of view.

This commit lacks two things:

1. Proper flag for DAG_id_tag_update(), so currently the only way
to inform depsgraph about nered to flush visiblity/selectability
flags is to tag scene ID for update. Flag of 0 will to the trick
for now.

2. Any operation which changes layer collections or render layers
need to tag relations to update. This is not done, so any change
to those will cause crash.