Adding Eigen, GLFW, ImGui & glad as submodules.

Former-commit-id: 2df6714623b9d2d2a8450f197e5ec4b877965500

.gitmodules

@@ -24,3 +24,12 @@
 [submodule "external/cork"]
 	path = external/cork
 	url = https://github.com/libigl/cork.git
+[submodule "external/imgui"]
+	path = external/imgui
+	url = https://github.com/ocornut/imgui.git
+[submodule "external/glfw"]
+	path = external/glfw
+	url = https://github.com/glfw/glfw.git
+[submodule "external/eigen"]
+	path = external/eigen
+	url = https://github.com/eigenteam/eigen-git-mirror.git