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
5a6ec0f003