Fixed failed sprite loading.

Now SIEGE returns NULL when a sprite has failed to load.
SG_EXPORT should be __cdecl if defined(__WIN32).
Added SG_CALL, deprecated SG_EXPORT.
Document SGBank usage with an example from the blog.
Merge branch 'master' of github.com:SIEGE/siege
Fixed a memory leak in the Set example.
Minor optimization in the set example.
Fixed a bug in 3D cross product in vector.c
Fixed some warnings and a compilation problem.

The warnings were generated by the CLang compiler (GCC seems to have missed the problems), while the compilation problem appeared in FreeBSD due to the inclusion of <malloc.h> in stb_vorbis.c.
Disabled the [broken] JSON parser for now
Made the debug overlay in the physics demo a tad less ugly