Geometry Nodes: support instance attributes when realizing instances

This patch refactors the instance-realization code and adds new functionality.
* Named and anonymous attributes are propagated from instances to the
  realized geometry. If the same attribute exists on the geometry and on an
  instance, the attribute on the geometry has precedence.
* The id attribute has special handling to avoid creating the same id on many
  output points. This is necessary to make e.g. the Random Value node work
  as expected afterwards.

Realizing instance attributes has an effect on existing files, especially due to the
id attribute. To avoid breaking existing files, the Realize Instances node now has
a legacy option that is enabled for all already existing Realize Instances nodes.
Removing this legacy behavior does affect some existing files (although not many).
We can decide whether it's worth to remove the old behavior as a separate step.

This refactor also improves performance when realizing instances. That is mainly
due to multi-threading. See D13446 to get the file used for benchmarking. The
curve code is not as optimized as it could be yet. That's mainly because the storage
for these attributes might change soonish and it wasn't worth optimizing for the
current storage format right now.

```
1,000,000 x mesh vertex:       530 ms -> 130 ms
1,000,000 x simple cube:      1290 ms -> 190 ms
1,000,000 x point:            1000 ms -> 150 ms
1,000,000 x curve spiral:     1740 ms -> 330 ms
1,000,000 x curve line:       1110 ms -> 210 ms
10,000 x subdivided cylinder:  170 ms ->  40 ms
10 x subdivided spiral:        180 ms -> 180 ms
```

Differential Revision: https://developer.blender.org/D13446

1 files changed, 1 insertions, 1 deletions

diff --git a/source/blender/blenkernel/BKE_blender_version.h b/source/blender/blenkernel/BKE_blender_version.h
index 83429b58faf..d0ab8be9a29 100644
--- a/source/blender/blenkernel/BKE_blender_version.h
+++ b/source/blender/blenkernel/BKE_blender_version.h
@@ -39,7 +39,7 @@ extern "C" {
 
 /* Blender file format version. */
 #define BLENDER_FILE_VERSION BLENDER_VERSION
-#define BLENDER_FILE_SUBVERSION 4
+#define BLENDER_FILE_SUBVERSION 5
 
 /* Minimum Blender version that supports reading file written with the current
  * version. Older Blender versions will test this and show a warning if the file