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
- add BLI_stack_count
- add BLI_stack_pop_n to pop into an array
- add BLI_stack_push_r, which returns a pointer that can be filled in
Also remove sanity check in BLI_stack_pop, assert if the stack is empty.