GDALCreateApproxTransformer2 needs cleanup

[Kurt Schwehr]

Based on review comments from pklier:

• It should not return a void *. That casting should be happening as a cast in GDALCreateApproxTransformer
• Why forward declare GDALCreateApproxTransformer2? Just reorder so that it only needs the static function

Should this comment say "*destination*?

/* Handle optional source approximation transformer.                    */
/* -------------------------------------------------------------------- */
    if( psInfo->pDstTransformer

[Even Rouault]

In 38786:

Fix comment (fixes #6902)

[Even Rouault]

I tried to move GDALCreateApproxTransformer2 upper, but it depends on other static methods that are defined lower, so this would imply moving too many things. Not really worth it IMHO

Regarding the return type, as all Create...Transformer() function return opaque pointers, this is consistant to do so.