Support "horizontal" previews - previews w/ inverted ratio (bug 871654)

https://bugzilla.mozilla.org/show_bug.cgi?id=871654

This is a simple fix to support "horizontal" screenshots. A more complex approach is discussed in https://bugzilla.mozilla.org/show_bug.cgi?id=900296, this bug and pull request is simply about supporting an inverted ratio in the meantime.

It looks like no Fireplace modifications are necessary.