Fix writing of new RIFF chunks at even positions

If the last chunk had an odd size, the new chunk would have been written at
odd position, which is incorrect.

This is based on the patch by Jens Dyffort, but I ended up changing the
implementation to correctly handle subsequential updates to the file.

The whole RIFF code really needs to be rewritten in a different way...

BUG:243954


git-svn-id: svn://anonsvn.kde.org/home/kde/trunk/kdesupport/taglib@1220223 283d02a7-25f6-0310-bc7c-ecb5cbfe19da