For cleanliness the prelude module only needs to reexport preludes from
direct, "top-most" crates, which themselves take care of reexporting
preludes from its dependencies again. This shaves off some code while
maintaining the same set of exports.
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
