Add subdirectories to source file collection logic in SConstruct up to 3 layers of depth

Added automatic file collection using Glob so that source files in directories that are up to 3 layers of depth into src/ get properly detected by SConstruct.

SConstruct

@@ -38,7 +38,11 @@ Run the following command to download godot-cpp:
 env = SConscript("godot-cpp/SConstruct", {"env": env, "customs": customs})
 
 env.Append(CPPPATH=["src/"])
+
 sources = Glob("src/*.cpp")
+sources.extend(Glob("src/*/*.cpp"))
+sources.extend(Glob("src/*/*/*.cpp"))
+sources.extend(Glob("src/*/*/*/*.cpp"))
 
 if env["target"] in ["editor", "template_debug"]:
     try: