Use more correct boudning rectangle in SparkOverlay.pointInOverlay() + nits

In particular, this fixes the problem where a click on a <button> or other plain HTML element in spark-overlay would close it (with autoCloseDisabled == false).
Merge pull request #774 from ussuri/fix-pointInOverlay

Use more correct boudning rectangle in SparkOverlay.pointInOverlay() + nits