LP#1685840: Google Books Preview: responsive sizing

LP#1685840: Google Books Preview: responsive sizing

Use the viewport's actual height and width to display the book preview,
rather than hard-coding 600px x 800px. On a very wide screen, the book
will be centred; on a small screen such as a mobile device, the book
will not scroll indiscriminately off to the side, but instead stays
within the bounds of the viewport.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Ben Shum <ben@evergreener.net>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>