Support for page progression independent of script writing-direction

This is referenced in http://www.daisy.org/epub/issues/epub-needs-support-vertical-directionality-writing but is not necessarily the same issue.

While it is true that in many languages, writing direction is correlated with the flow of pages, they aren't necessarily synonymous. For example, manga in Japanese has artwork that is meant to be viewed right-to-left (in keeping with right-left/vertical text direction). When the same manga is translated into, for example, English, the writing direction becomes left-right/horizontal but the artwork constraints mean the pages are still bound right-to-left.

The CSS3 Paged Media specification (http://dev.w3.org/csswg/css3-page/#progression) has some discussion of this:

The page progression direction is determined as follows:

• If text is laid out in horizontal lines, the page progression is the same as the inline progression.
• If text is laid out in vertical lines, the page progression is the same as the block progression.

However, that's not suitable for the case described above. It should be possible to specify page-binding-direction independently of the directionality of the script.