gegl_malloc() stores a pointer to the real malloc-ed buffer in front
of the returned buffer, requiring sizeof(gpointer) bytes on top of
GEGL_ALIGN.  Since the value of GEGL_ALIGN (16) easily fits in a
single byte, we can replace this pointer with a single-byte offset
to the real start of the buffer.  Since the offset from the real
start of the buffer to the returned buffer is at least 1, we don't
need to allocate any additional space on top of GEGL_ALIGN.