Add a cmake add_subdirectory for scene_renderer/shaders (#470)

Since shaders are already checked in.
This line is taken out of this PR:
#427

assets/CMakeLists.txt

@@ -20,3 +20,4 @@ add_subdirectory(fluid_simulation/shaders)
 add_subdirectory(gbuffer/shaders)
 add_subdirectory(materials/shaders)
 add_subdirectory(oit_demo/shaders)
+add_subdirectory(scene_renderer/shaders)