Rewrite on simdjson with a flat src and smack-style build

.gitmodules

@@ -1,18 +1,14 @@
-[submodule "external/SDL"]
-	path = external/SDL
-	url = https://github.com/libsdl-org/SDL
-[submodule "external/SDL_mixer"]
-	path = external/SDL_mixer
-	url = https://github.com/libsdl-org/SDL_mixer
 [submodule "external/imgui"]
 	path = external/imgui
 	url = https://github.com/ocornut/imgui
-[submodule "external/glm"]
-	path = external/glm
-	url = https://github.com/g-truc/glm
-[submodule "external/tinyxml2"]
-	path = external/tinyxml2
-	url = https://github.com/leethomason/tinyxml2
-[submodule "external/physfs"]
-	path = external/physfs
-	url = https://github.com/icculus/physfs
+[submodule "external/sdl"]
+	path = external/sdl
+	url = https://github.com/libsdl-org/SDL.git
+	branch = release-3.4.x
+[submodule "external/stb"]
+	path = external/stb
+	url = https://github.com/nothings/stb.git
+[submodule "external/simdjson"]
+	path = external/simdjson
+	url = https://github.com/simdjson/simdjson
+	shallow = true