Better sync of thumbnails with seek bar + thumbnails while scrubbing even on mouseout

[jcaron23]

These two commits are for the following changes:

• better sync of thumbnails with seek bar: the position where the plugin would switch from one thumbnail to the next was not in sync with the seek bar current position. For instance, in the sprite example, it would not switch precisely at 0:10 as it was supposed to, but somewhere a bit further, nearly 0:11.
  This was due to 2 issues: the computation of the seek position did not take into account the size of the handle like videojs's code does, and the use of a strict > instead of >= when looking for a matching thumbnail.
• display thumbnails while scrubbing even on mouse out: when you click on the seek bar and keep the mouse down to scrub through the video, you needed to keep the mouse within the seek bar to continue to see thumbnails. These changes keep the thumbnails visible even if you move the mouse out of the seek bar.