Fix calculation of pane width

Fix calculation of pane width with respect to element padding.

- jQuery `innerWidth` includes element padding, so adding padding to innerWidth value gives incorrect pane width.
- jQuery `width` does not include element padding, so no need to substract padding width from `width`.

This pull request addresses the same main issue as https://github.com/vitch/jScrollPane/pull/208, which currently seems to be incomplete and introduces syntax errors.