Fix scroll offset calculation when <regular-table> has non-zero padding

[texodus]

This PR fixes a scroll issue when non-zero padding is applied to the regular-table element. It is based on #241 (thanks @timkpaine !) but memoizes the potentially expensive clientHeight read and fixes scrollToCell reciprocal calculation.

In general, its best to avoid applying layout CSS to regular-table itself as it may interfere with the library's geometry calculation. In the case of padding, there were a few issues to resolve:

• The visible area of the table is now smaller than the scrollable area which doesn't really have a regular DOM corollary (note the scrollable area and scrollbar in green, viewable area in red). This must be corrected everywhere we calculate virtual scrollable area or viewport size:
  
• In addition to padding and margin, the viewable area must accomodate for the scrollbar gutter size when the viewable area and the scrollable area are the same (or within the margin of the gutter size).