Added demo writer interface, restructure

external/sourcesdk/valve_support.h

@@ -2,6 +2,7 @@
 #pragma once
 
 #include <cstdint>
+#include <cstddef>
 #include <cassert>
 #include "vector.h"
 
@@ -25,8 +26,6 @@ using byte = char;
     #define PLATFORM_WINDOWS_PC64 1
 #endif
 
-#define NULL nullptr
-
 #define Assert(x) assert(x)
 #define AssertMsg(x, ...) assert(x)
 #define AssertMsg2(x, ...) assert(x)