Provide an appropriate fallback during Import of some incorrect Alembic
curve data and write out valid data during our Export as well.
The buggy file in question has 2 problems:
- The curve data is marked as being `kVariableOrder` but provides no
actual `order` data. This is invalid according to Alembic code [1]
- The curve data is also marked as being `kBezierBasis` but the actual
values are most likely not bezier judging from how many elements are
in each curve array
This PR focuses on the first problem and allows the file to load
"correctly" - or at least as correctly as it did prior to Blender 4.2.
If we encounter such a file, we will fallback to interpreting the data
as linear POLY curves. It also ensures that we only use `kVariableOrder`
when needed during Export.
[1] 23f1c76d7c/lib/Alembic/AbcGeom/ICurves.h (L91)
Pull Request: https://projects.blender.org/blender/blender/pulls/126378
ce82d4434f