`CellMeasurer` implementation changed to use `ReactDOM.unstable_renderSubtreeIntoContainer` instead of `ReactDOMServer.renderToString` in order to support `context`.
`Grid` has been changed slightly as well to calculate its visible children just before `render` (instead of in it).
This change is not expected to have any public-facing consequences beyond supporting the `context` property for `CellMeasure`d cells.
Thanks to @jquense for this contribution!
