Make proper screenshots for underlying scrollable views

When showing menu above UITableView (or other scrollable view) screenshot is rendered incorrectly if table is scrolled down from the top.
![rngridmenuscreenshot](https://f.cloud.github.com/assets/2015453/858082/a88c093e-f55e-11e2-8c20-3dda434f7ffe.png)

For such case we need to make screenshot based on current table content offset. Added such ability.