nutenc: avoid av_tree_find() operation per syncpoint

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/nutenc: add mode that omits the index

When the index is not written, several data tables become unneeded,
reducing memory and cpu requirements.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/nutenc: limit index table size if no index is going to be written

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>