Update SDL2 and move to using submodules

.gitmodules

@@ -1,6 +1,39 @@
 [submodule "civetweb"]
 	path = civetweb
 	url = https://github.com/splashkit/civetweb
+	shallow = yes
 [submodule "serial"]
 	path = serial
 	url = https://github.com/wjwwood/serial.git
+	shallow = yes
+[submodule "SDL"]
+	path = SDL
+	url = https://github.com/libsdl-org/SDL.git
+	branch = release-2.30.x
+	shallow = yes
+[submodule "SDL_mixer"]
+	path = SDL_mixer
+	url = https://github.com/libsdl-org/SDL_mixer.git
+	branch = release-2.8.x
+	shallow = yes
+[submodule "SDL_net"]
+	path = SDL_net
+	url = https://github.com/libsdl-org/SDL_net.git
+	branch = release-2.2.x
+	shallow = yes
+[submodule "SDL_ttf"]
+	path = SDL_ttf
+	url = https://github.com/libsdl-org/SDL_ttf.git
+	branch = release-2.20.x
+	fetchRecurseSubmodules = no
+	shallow = yes
+[submodule "SDL_image"]
+	path = SDL_image
+	url = https://github.com/libsdl-org/SDL_image.git
+	branch = release-2.6.x
+	fetchRecurseSubmodules = no
+	shallow = yes
+[submodule "libpng"]
+	path = libpng
+	url = https://github.com/pnggroup/libpng.git
+	shallow = yes