Add SDL2 source as a submodule

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "3rd/googletest"]
 	path = 3rd/googletest
 	url = https://github.com/google/googletest
+[submodule "3rd/SDL"]
+	path = 3rd/SDL
+	url = https://github.com/sdlpal/SDL.git