Build: Enable unity build for bf_compositor

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

source/blender/compositor/CMakeLists.txt

@@ -645,6 +645,11 @@ endif()
 
 blender_add_lib(bf_compositor "${SRC}" "${INC}" "${INC_SYS}" "${LIB}")
 
+if(WITH_UNITY_BUILD)
+  set_target_properties(bf_compositor PROPERTIES UNITY_BUILD ON)
+  set_target_properties(bf_compositor PROPERTIES UNITY_BUILD_BATCH_SIZE 10)
+endif()
+
 if(COMMAND target_precompile_headers)
   target_precompile_headers(bf_compositor PRIVATE COM_precomp.h)
 endif()