The curve interpolation operator write uninitialized data to vertex
attributes if there is more than one interpolated curve pair.
This is because _partial writes_ to `VArraySpan` wrappers only work
if the original VArray is already a span or if the wrapper span is
fully initialized with the original data beforehand.
In the case of the curve interpolation tool for Grease Pencil the
interpolation is invoked for each curve pair separately, creating a
new output attribute `VArraySpan` wrapper each time. This wrapper
is only filled for the curve pair in question and writes uninitialized
data to all the other curves' vertex weight attributes.
To prevent this from happening the simple solution is to use
`lookup_or_add_for_write_span` which initializes the entire span.
This causes quite a lot of unnecessary copying, but that is acceptable
for the Grease Pencil interpolation tool. The alternative is to change
the tool so that the destination GSpanAttributeWriter is only created
once, but that is a much bigger change.
Pull Request: https://projects.blender.org/blender/blender/pulls/140283
a25e1c9267