ignore-inactive-pads on glvideomixer (or force live mode)

[Michiel Konstapel]

Is there a way to set ignore-inactive-pads on glvideomixer, like 
GstAudioAggregator has? Or another way to force it into live mode before 
a live source is actually attached? I would like to start it off with 
just a repeat-on-eos input (placeholder image), and then attach the live 
source when it connects, but unless the mixer thinks/knows it's live, it 
would just produce an unlimited number of buffers from the repeat-on-eos 
input (if I understand correctly).

Cheers,
Michiel