With this patch, materials are kept intact in simulation zones and bake nodes
without any additional user action.
This implements the design proposed in #108410 to support referencing
data-blocks (only materials for now) in the baked data. The task also describes
why this is not a trivial issue. A previous attempt was implemented in #109703
but it didn't work well-enough.
The solution is to have an explicit `name (+ library name) -> data-block`
mapping that is stored in the modifier for each bake node and simulation zone.
The `library name` is necessary for it to be unique within a .blend file. Note
that this refers to the name of the `Library` data-block and not a file path.
The baked data only contains the names of the used data-blocks. When the baked
data is loaded, the correct material data-block is looked up from the mapping.
### Automatic Mapping Generation
The most tricky aspect of this approach is to make it feel mostly automatic.
From the user point-of-view, it should just work. Therefore, we don't want the
user to have to create the mapping manually in the majority of cases. Creating
the mapping automatically is difficult because the data-blocks that should
become part of the mapping are only known during depsgraph evaluation. So we
somehow have to gather the missing data blocks during evaluation and then write
the new mappings back to the original data.
While writing back to original data is something we do in some cases already,
the situation here is different, because we are actually creating new relations
between data-blocks. This also means that we'll have to do user-counting. Since
user counts in data-blocks are *not* atomic, we can't do that from multiple
threads at the same time. Also, under some circumstances, it may be necessary to
trigger depsgraph evaluation again after the write-back because it actually
affects the result.
To solve this, a small new API is added in `DEG_depsgraph_writeback_sync.hh`. It
allows gathering tasks which write back to original data in a synchronous way
which may also require a reevaluation.
### Accessing the Mapping
A new `BakeDataBlockMap` is passed to geometry nodes evaluation by the modifier.
This map allows getting the `ID` pointer that should be used for a specific
data-block name that is stored in baked data. It's also used to gather all the
missing data mappings during evaluation.
### Weak ID References
The baked/cached geometries may have references to other data-blocks (currently
only materials, but in the future also e.g. instanced objects/collections).
However, the pointers of these data-blocks are not stable over time. That is
especially true when storing/loading the data from disk, but also just when
playing back the animation. Therefore, the used data-blocks have to referenced
in a different way at run-time.
This is solved by adding `std::unique_ptr<bake::BakeMaterialsList>` to the
run-time data of various geometry data-blocks. If the data-block is cached over
a longer period of time (such that material pointers can't be used directly), it
stores the material name (+ library name) used by each material slot. When the
geometry is used again, the material pointers are restored using these weak name
references and the `BakeDataBlockMap`.
### Manual Mapping Management
There is a new `Data-Blocks` panel in the bake settings in the node editor
sidebar that allows inspecting and modifying the data-blocks that are used when
baking. The user can change what data-block a specific name is mapped to.
Pull Request: https://projects.blender.org/blender/blender/pulls/117043
Jacques Lucke