libIsoRender is not part of this repository

Building gives me this:

```
CMake Error at CMakeLists.txt:5 (add_subdirectory):
  add_subdirectory given source "libIsoRender" which is not an existing
  directory.
```

https://github.com/X123M3-256/libIsoRender should be submoduled.

*(The same issue exists for libImage within libIsoRender; potentially other instances, too)*