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
There's a VideoTimeCode type that represents any kind of timecodes,
including invalid ones, and which allows to change each field
individually. And ValidVideoTimeCode that has all fields
validated and that can be used with most of the API.
In C, validation of the timecodes is left to the user and most functions
assert on getting passed valid timecodes.
