Shader Nodes: Unity Build

Enable unity builds for  `bf_nodes_shader`, gives about a 2.7x speedup
of total compile times when just building `bf_nodes_shader`.
On my machine, this equates to saving about 30 seconds.

Differential Revision: https://developer.blender.org/D13720

source/blender/nodes/shader/CMakeLists.txt

@@ -166,3 +166,8 @@ if(WITH_FREESTYLE)
 endif()
 
 blender_add_lib(bf_nodes_shader "${SRC}" "${INC}" "${INC_SYS}" "${LIB}")
+
+if(WITH_UNITY_BUILD)
+  set_target_properties(bf_nodes_shader PROPERTIES UNITY_BUILD ON)
+  set_target_properties(bf_nodes_shader PROPERTIES UNITY_BUILD_BATCH_SIZE 10)
+endif()