sd_lavc: remove ineffective code

It makes no sense to set the packet duration, because libavcodec doesn't
know the timebase. And in fact, no subtitle decoder accesses the packet
duration, except text subtitle converters, which are not relevant here.
So this code did nothing - drop it.

Also fix a blatantly incorrect comment.
demux: avoid unnecessary wakeups

If a packet is appended to a stream, and there were already packets
queued, nothing about the state changed, as far as the user (i.e. the
player) is concerned. Thus no wakeup is needed.

The pthread_cond_signal() call following this is not interesting - it
will simply be a NOP if there are actually no waiters.
win32: correct HANDLE type

The correct type is HANDLE, not HANDLE*, though this change shouldn't
affect functionality.
win32: correct SGR sequence handling

This should get colour working again on the Windows console.

Fixes #1032.