All parts of drawing (shaders, GL mesh descriptor, material partitioner
and so on) needs to be redone for the draw manager and new OpenSubdiv
library.
Removing untested code which is doomed to be replaced to make localized
refactoring easier.
Apply clang format as proposed in T53211.
For details on usage and instructions for migrating branches
without conflicts, see:
https://wiki.blender.org/wiki/Tools/ClangFormat
C-API is way smaller than the rest of the code which uses it.
So better to conditionally compile stub implementation than
to keep adding ifdef everywhere.