Previously the UV unwrapping handling for subsurf modifiers used
`DerivedMesh`to implement the subdivision. Since we're trying to remove
`DerivedMesh` in general, and since this just made use of the `Mesh`
data anyway, it's relatively simple to remove it here. Combined with
D15939, this makes it possible to remove more `DerivedMesh` code.
Differential Revision: https://developer.blender.org/D16487
This commit replaces the `Mesh_Runtime` struct embedded in `Mesh`
with `blender::bke::MeshRuntime`. This has quite a few benefits:
- It's possible to use C++ types like `std::mutex`, `Array`,
`BitVector`, etc. more easily
- Meshes saved in files are slightly smaller
- Copying and writing meshes is a bit more obvious without
clearing of runtime data, etc.
The first is by far the most important. It will allows us to avoid a
bunch of manual memory management boilerplate that is error-prone and
annoying. It should also simplify future CoW improvements for runtime
data.
This patch doesn't change anything besides changing `mesh.runtime.data`
to `mesh.runtime->data`. The cleanups above will happen separately.
Differential Revision: https://developer.blender.org/D16180
