Use stb_rect_pack.h for Pack operations #5

Hi, could it be possible to update/regenerate the code with stb_rect_pack.h included?

From my understanding, when stb_rect_pack.h is not available, the Pack operations uses a suboptimal code path (see here, here and here).

Since the C# code is generated from the C++ code, the different code paths are no longer available here, so including e.g. StbRectPackSharp will not work.