nle: Seek the whole stack on initialization

nle: Seek the whole stack on initialization

Instead of seeking each nleobject separately to setup new stack, wait
for the whole stack to preroll and then seek that newly setup stack,
leading to the same code path and seek 'tweaking' as when processing
a seek on the composition (without stack changes).

This is mandatory to properly handle filter that tweak segments to handle
time remapping for example.