Fix RNA not giving error with invalid identifiers for collection properties

Fix lost node links when linked node group datablock is temporarily missing

Don't refresh the list of sockets, so that when the .blend file is restored the
links remain valid. Also display such nodes in red to indicate an error, same
as when the node type info is missing.

Merge branch 'blender-v2.91-release'

Fix T82673: Cycles crash with zero emission strength and linked emission color

Merge branch 'blender-v2.91-release'

Fix T82607: crash cancelling Cycles render during adaptive subdivision update

Now that the Blender sync mechanism deletes nodes from the scene, we need to
ensure scene update is stopped before we do this.

Also add some more early out in scene geometry update to ensure we do not
continue working on incomplete geometry data, though that was not the cause of
this crash.

Cleanup: compiler warning

Cleanup: remove accidentally committed merge files

Fix T82516: Cycles crash updateding animated volumes after NanoVDB

Two issues:
* Automatic deduplication of OpenVDB grid data was failing when Cycles had
already cleared the OpenVDB grid, causing an empty grid. Instead rely on
Blender return the same OpenVDB grid pointer when deduplication is possible.
* The volume bounds mesh was not properly cleared when the OpenVDB grid was
empty, causing a mismatch between mesh and voxel data.

Fix empty Cycles render devices panel showing in preferences on macOS

There is no GPU rendering support on macOS, so showing the panel only adds
confusion. Also hide the panels in builds without Cycles.