The issue was that boolean custom data layers were not written to files,
because the dna struct name `bool` does not exist. Adding a struct that
just contains a `bool/uint8_t` does not seem to be possible, it looks like
the minimum dna struct size is 4 bytes.
The proposed solution has two parts:
1. Write the custom data layer using `BLO_write_raw` instead of
`BLO_write_struct_array_by_name`.
2. When loading a file, reinitialize any custom data layer that was
not saved correctly (this is just a fix for existing files).
Differential Revision: https://developer.blender.org/D10194
dc8b31af57