Size to use for all other items
Array of sizes to use for the initial items, one size per item
Offset from start of container to specified item
itemOffset(n) should be equal to Sum{i:0->n-1}(itemSize(i))
To efficiently support large containers, cost should be O(logn) or better.
Size of item with given index
Given an offset, return the index of the item that intersects that offset, together with the start offset of that item
Implementation of ItemOffsetMapping for use when initial items have variable sizes.