make stb_vorbis_alloc* parameter in APIs be const

minor tweak to get_samples_interleaved documentation

dummy definitions for malloc et al (note you have to modify source to make this work though anyway);
tweak credits change;

fix a few crash bugs with invalid stb_vorbis files (reported by Philip Bennefall, found using a fuzz tester)

test program for seeking (stb)

mingw redefines __forceinline (though I don't think this applies to all
versions of mingw.) Therefore, don't redefine it if mingw has already
defined it.

"const" added in stb_vorbis_decode_memory and stb_vorbis_open_memory to
allow to read from const buffers.

Conflicts:
	stb_vorbis.c

I manually made these changes because they
disabled some substantial amounts of code,
and I wanted to make sure this wasn't due to
original code having bugs or deviating from
the specification

QSORT only takes CDECL, but project settings can define different defaults for calling conventions.