Silence a few Xcode warnings

Using while loops for the tile size modification makes more sense
anyway, as does the column/row initialization order. The dangling else
when setting idxInMax has also been tied off.
Eliminate unnecessary calling convention keywords

Longstanding oddity left over from the first released version, when
there was only GetFrame. Became extraneous as soon as I split up
interleaved/planar processing; somehow it's escaped my attention all
this time.