Merge branch 'master' into geometry-nodes

Geometry Nodes: do not create geometry depsgraph relation for empty object

This resulted in a warning.

Geometry Nodes: support muted nodes

The handling of muted nodes is handled at the derived node tree
level now. This is also where expanding node groups is handled.
Muted nodes are relinked and removed from the derived tree
during construction. The geometry node evaluation code does
not have to know about muted nodes this way.

Geometry Nodes: fix when vertex group exists but there are no MDeformVerts yet

The `mesh->dvert` array is only allocated when needed. Just creating a new vertex
group without any vertices in it, does not trigger the allocation.

Geometry Nodes: use attribute span api in Join Geometry node

Copying over attributes from the original geometries to the new
geometry only takes ca. a third of the time compared to before.

Geometry Nodes: use attribute span api in random attribute node

This results in a ~30% speedup. The execution time is bounded
by the random number generator here.

Cleanup: correct comment

Geometry Nodes: use attribute span api in attribute math node

For a simple add operation, this results in a 3-4x speedup.

Geometry Nodes: support accessing attributes with spans

Before, attributes could only be accessed via setter and getter functions.
While this works well, it can be cumbersome and slow.
It would be better if most code could just operate on the underlying
attribute array directly. The issue is that some attributes are currently
not stored in plain arrays (e.g. vertex positions, vertex groups, ...).

This patch implements a solution where these "special" attributes are
converted into a temporary array for easy access and then copied
back into their correct form after all modifications are done.

Attribute accessors now have a `get_span` and `apply_span` method,
which can be used exactly for that purpose. In the case of attributes
that are plain arrays internally, only small constant overhead is added.

Geometry Nodes: protect object from instancing itself directly

This is not a perfect check, because it does not check instances recursivly.
However, it helps protect us from the most common mistake.