Fix comments that got search-and-replaced incorrectly

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

Conflicts:
	stb_image.h

also updated the comment at the top of the file to reflect stbi_write_tga's new capabilities.

For some reason Microsoft CL compiler for ARM is unable to compile malloc when parameter is an expression. malloc(x * y) will cause internal compiler error, malloc(x) is however fine.

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