- 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.
BLI_buffer is a dynamic homogeneous array similar to BLI_array, but it
allocates a structure that can be passed around making it possible to
resize the array outside the function it was declared in.
