Commit ba2e500b ensured to provide
a running clock when EOS had finished rendering.  However,
other measures are needed (and were in place before) to ensure a
running clock when EOS still needs rendering (i.e. waiting).

So, specifically, re-introduce eos_rendering removed in aforementioned commit,
this time as a public variable so subclasses can be aware of the situation.

Fixes (part of) #645961.

API: GstBaseAudioSink:eos_rendering