update linked version of eigen to 3.2.4

libigl now uses the fork of nanogui in the libigl github


Former-commit-id: 95efea1c157e45572f1b66be5a81110a8c2c135c

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "external/nanogui"]
 	path = external/nanogui
-	url = https://github.com/schuellc/nanogui.git
+url=https://github.com/libigl/nanogui.git
         fetchRecursiveSubmodules = true
         ignore = dirty