fetch recursive

Former-commit-id: 190f7ff12ffd64a94452c5958ba0cf84ce013040

.gitmodules

@@ -1,3 +1,4 @@
 [submodule "external/nanogui"]
 	path = external/nanogui
 	url = https://github.com/schuellc/nanogui.git
+        fetchRecursiveSubmodules = true