Quick hack to use a single shared array for results from multiple Unity OverlapCollider calls

CombinedOverlapCollider.cs

    // I always use the Unity collider calls which place results in a single pre-allocated array to avoid allocations.
    // However it's a bit annoying that you can't make a call like `collider2D.OverlapCollider` and tell it to start
    // inserting results at a non-zero index in that results array, in order to collect multiple tests in a single
    // result array.
    
    // Yes, you can (and should) use CompoundCollider2D if this is a single object you're tracking, but if it's not and
    // you just want combined results from different colliders it's a ball-ache to not be able to get all results in a
    // single array easily.
    
    // This little snippet shunts results of earlier OverlapCollider calls to the end of the array temporarily,
    // then recombines them all at the end so the effect is the same as being able to use an offset in the call.

    public int OverlapCollider(ContactFilter2D filter, Collider2D[] results) {
        int totalCount = 0;
        int stashed = 0;
        for (int i = 0; i < colliders.Length && totalCount < results.Length; ++i) {
            var coll = colliders[i];
            int count = coll.OverlapCollider(filter, results);
            if (count > 0 && colliders.Length > 1) {
                // Unfortunately Unity doesn't allow an offset to OverlapCollider, so stash previous results
                // at the end of the array until we're done
                
                // truncate if need be, should never be exactly zero since outer loop would abort first 
                if (count + stashed > results.Length)
                    count -= results.Length - stashed;
                
                if (i == colliders.Length - 1) {
                    // this was the last one, so recombine the stashed ones (ordering is changed compared to colliders but no biggie)
                    Array.Copy(results, results.Length - stashed, results, count, stashed);
                    // free refs at end too
                    Array.Clear(results, results.Length - stashed, stashed);
                } else {
                    // Stash current results at the end
                    Array.Copy(results, 0, results, results.Length - stashed - count, count);
                    stashed += count;
                }        
            }
            totalCount += count;
        }
        return totalCount;
    }