winealsa: Avoid underrun by adding a lead-in when starting with few samples.

[Joerg-Cyril.Hoehle at t-systems.com]

Hi,

for reference, see #29299.

The lead-in does not suffice to prevent underruns.  More precisely,
invoking the callbacks timely is the one essential requirement to
prevent underruns.  The lead-in merely ensures that ALSA has something
to do even when GetCurrentPadding remains 0 for one period.

The lead-in may solve half the bugs about apps hanging when writing
few samples.  The other half depends on the lead-out, i.e. both
GetCurrentPadding and GetPosition reaching the sum of Released frames.

Regards,
	Jörg Höhle
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