Generally we don't want to do per-element operations on these spans
because of the overhead of the runtime type system, but these operations
on the whole span avoid ugly pointer arithmetic in other areas.
`GSpan` and spans based on virtual arrays were not default constructible
before, which made them hard to use sometimes. It's generally fine for
spans to be empty.
The main thing the keep in mind is that the type pointer in `GSpan` may
be null now. Generally, code receiving spans as input can assume that
the type is not-null, but sometimes that may be valid. The old #type() method
that returned a reference to the type still exists. It asserts when the
type is null.
This commit re-implements the resample curve node to use the new curves
type instead of CurveEval. The largest changes come from the need to
keep track of offsets into the point attribute arrays, and the fact
that the attributes for all curves are stored in a flat array.
Another difference is that a bit more of the logic is handled by
building of the field network inputs. The idea is to let the field
evaluator handle potential optimizations while making the rest of the
code simpler.
When resampling 1 million small poly curves,the node is about 6
times faster compared to 3.1 on my hardware (500ms to 80ms).
This also adds support for Catmull Rom curve inputs.
Differential Revision: https://developer.blender.org/D14435
