`CellMeasurer` passes `index` param (duplicate of `rowIndex`) in order to more easily integrate with `List` by default.

`Grid` now better handles a mix of controlled and uncontrolled scroll offsets. Previously changes to one offset would wipe out the other causing cells to disappear (see PR #482). This is an edge-case bug and only impacted an uncommon usecase for `Grid`. As such this change is expected to only impact only a small percetange of users.
