extend BLI_buffer
- add option to calloc or not, existing code wasnt consistent here, would calloc on first alloc but not on realloc, also - the static memory was never zero'd.
  use flag BLI_BUFFER_USE_CALLOC to ensure all new memory is zero'd (static/alloc/realloc's).

- add BLI_buffer_declare_static / BLI_buffer_declare so its possible to have a buffer that never uses static memory.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53503 954f8c5b-7b00-dc11-b283-0030488c597c