Bug 66287: Proper caption styling

* Make sure borders render properly even in Chrome & with zoom on high-dpi
  devices. overflow: auto is important for this.

* Set borders & background on imagecaption

* Enable border-collapsing; we are rendering this as a table & want the
  caption & figure to show a single border

* Add appropriate margin on the image to create some padding inside the figure
  despite border-collapsing

Tested in Chrome & Firefox.

Change-Id: Ibb46b682138ac703bea364efaeefc376e60c2d5f