added external

diff --git a/external/gl3w b/external/gl3w
new file mode 160000 (submodule)
index 0000000..8c727cd
--- /dev/null
+++ b/external/gl3w
@@ -0,0 +1 @@
+Subproject commit 8c727cd02803aa05783abc56eb212fb26d4c205d

diff --git a/external/imgui b/external/imgui
new file mode 160000 (submodule)
index 0000000..bf83ac8
--- /dev/null
+++ b/external/imgui
@@ -0,0 +1 @@
+Subproject commit bf83ac8b555d851a0bba3707e633c33923cfa54a