Part of #116901.
The BMesh version of the algorithm was parallelized because BMesh has
edge to face topology lookup available.
This also allows removing `GPU_vertbuf_steal_data`, by just generating
the correct data type in the first place. Removing that should help dealing
with improvements to VBO data ownership in the future.
Now that all relevant code is C++, the indirection from the C struct
`GPUVertBuf` to the C++ `blender::gpu::VertBuf` class just adds
complexity and necessitates a wrapper API, making more cleanups like
use of RAII or other C++ types more difficult.
This commit replaces the C wrapper structs with direct use of the
vertex and index buffer base classes. In C++ we can choose which parts
of a class are private, so we don't risk exposing too many
implementation details here.
Pull Request: https://projects.blender.org/blender/blender/pulls/119825
