* Added nullability attributes to AVFoundation
* Revert AVPlayerItem FromAsset nullability attributes changes to keep existing tests passing and allow backwards compatibility
* Documents common-AVFoundation.ignore entry related to Foundation.NSNumber[] AVFoundation.AVVideoCompositionInstruction::get_RequiredSourceTrackIDs()
!extra-null-allowed! 'Foundation.NSNumber[] AVFoundation.AVVideoCompositionInstruction::get_RequiredSourceTrackIDs()' has a extraneous [NullAllowed] on return type
Enabling this will ensure that future bindings (and Xcode updates that
change nullability information) are spotted right away.
The binding fixes are **not** complete, i.e. what was done was mostly
to debug the xtro rule and find corner cases. The backlog will be
_ignored_ so the builds won't fail.