It does not borrow from anything else and can be safely stored away.
Only the VideoCodecState<InNegotiation> is actively borrowing from the
element to ensure that it is only modified during negotiation in a safe
way.
The VideoCodecFrame and VideoCodecState is C API is unfortunatelly unsafe
by design. So we workarounded it by ensuring the decoder stream lock was
hold while user has a writable reference on those objects.
Based on previous work from Thibault Saunier and Philippe Normand.
Fixes#161
