wng: Avoid buffer underruns due to filter chain output buffer constraints.

wng: Avoid buffer underruns due to filter chain output buffer constraints.

Using ogg vorbis streams together with the oss writer hits the following nasty bug:
In case the filter chain can provide more data than what fits into its output buffer,
it converts the maximal amount possible to completely fill its output buffer. However, the
time to play this data might be less than than the time until the next data packet arrives
from the upper layers, especially when using ogg vorbis streams and FEC.

Since the filter chain task has no pre_select function,  the convert function(s) of its filter
nodes only get the chance to convert more data until the next select call returns, which
might already be too late.

This patch fixes the bug by teaching the pre_select function of the writer node group
to  remember whether something was written during the previous call  to wng_post_select().
In this case we force a minimal timeout for select, i.e. the next call to select() will return
immediately and the convert functions of the filter node are called again, hopefully converting
more data for the writer node group.