At the time of allocating the buffer with vertices in context, we don't
know exactly how many vertices are affected, but we do know that it is
less than or equal to twice the number of vertices killed.
I had assumed that the span's size was the same as the length variable.
In the future, separate lengths could be removed in favor of using
lengths directly from spans.
In this commit I changed many loops to range-based for loops.
I also removed some of the redundant iterator variables, using
indexing inside the loop instead. Generally an optimizing compiler
should have no problem doing the smartest thing in that situation,
and this way makes it much easier to tell where data is coming from.
I only changed the loops I was confident about, so there is still more
that could be done in the future.
Differential Revision: https://developer.blender.org/D13637
This moves `MOD_weld.cc` to C++, fixing compiler warnings
coming from the change. It also goes a little bit further and converts
the code to use C++ data structures: `Span`, `Array`, and `Vector`.
This makes the code more shorter and easier to reason about, and
makes memory maneagement more automatic.
Differential Revision: https://developer.blender.org/D13618
