Blender's compositor code already makes extensive use of namespace which makes it very simple to enable unity build. There was one duplicated function that has since to be moved to a common header. I saw roughly a 3x speedup of bf_compositor using ninja on linux using i5 8250u (1:34 down to 0:34). Reviewed By: LazyDodo Differential Revision: https://developer.blender.org/D13792