struct Rav1dWarpedMotionParams: Make abcd: [i16; 4] field atomic

[kkysen]

This eliminates the only mutation of Rav1dFrameHeader, thus allowing it to be stored in an Arc without any borrowck problems.

This uses the atomig crate to make the [i16; 4] field atomic. The crate is quite simple, offering an Atomic<T> type that can only be implemented for actually lock-free atomic types through a simple pack and unpack into an atomic type (u64 in this case for [i16; 4]).

The alternatives would to use the atomic crate, which also offers an Atomic<T>, but doesn't guarantee a lock-free implementation, and whose implementation seems more complex. The other alternative would be to basically recreate what atomig is doing of packing and unpacking into an u64 for atomic operations, so I think using the existing atomig crate is simpler.

Update: I replaced atomig with an inline solution of about the same complexity.