Guillaume Desmottes authored May 11, 2019 and Sebastian Dröge committed May 22, 2019

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