This fixes a bug that happens when two drawings in the same grease pencil
data-block reference the same array. In this case, the same attribute array
pointer is written to .blend files for both drawings. However, when loading the
.blend file, it was not detected that the data is shared. Instead each drawing
would think that it was the single owner of the array even though it was shared.
The same problem can technically occur for all kinds of shared data, and not
just for grease pencil. However, only grease pencil shows this issue currently,
because it can easily happen that the same attribute is shared between different
`CustomData` within the same data-block. This very rarely happens with meshes or
curves, because different `CustomData` usually have different sizes and don't
share the same arrays.
A previous fix (cc891aa699) solved pretty much the same issues for the case
when two layers in the same `CustomData` share data. This original fix is now
generalized and is integrated with `BLO_read_shared` which makes it less error
prone. For each data-block, the `BlendDataReader` now remembers which shared
data it has loaded before so that it is not loaded again.
In the future this could be extended to support data that is shared between
data-blocks, but that's not as straight forward as one would hope currently. For
various reasons, different data-blocks could store independent data at the same
pointer. I specifically noticed that with a regression test file:
`blender/tests/data/modeling/geometry_nodes/attributes/attribute_statistics.blend`.
To support it, one solution could be to tag `BHead` in .blend files to indicate
that a specific pointer is shared and unique within the entire file. But that's
for another day.
Pull Request: https://projects.blender.org/blender/blender/pulls/122780
34588cd933