oggdemux: fix wrong first granule

oggdemux: fix wrong first granule

The code was using the first nonnegative granulepos to seed the
granule tracking, which appeared to work since headers have zero
granulepos. However, this does not work for files with a hole at
start, which are common in live streaming.

The correct behavior is to look for the first granule, and subtract
the duration of all the packets finishing on this page.

The function which does this relies on the fact that the ogg_stream
structure can be duplicated by shallow copy, in order to pull the
packets from the first page(s) on the copy without affecting the
original stream state.