ACTION-71: Behavior of absolute-position XSL-FO property

According to XSL-FO 1.1 Section 7.6.1 [1], the following apply:

absolute

The area's position (and possibly size) is specified with the "left",
"right", "top", and "bottom" properties. These properties specify offsets
with respect to the area's nearest ancestor reference area. Absolutely
positioned areas are taken out of the normal flow. This means they have no
impact on the layout of later siblings. Also, though absolutely positioned
areas have margins, they do not collapse with any other margins.
fixed

The area's position is calculated according to the "absolute" model, but in
addition, the area is fixed with respect to some reference. In the case of
continuous media, the area is fixed with respect to the viewport (and
doesn't move when scrolled). In the case of paged media, the area is fixed
with respect to the page, even if that page is seen through a viewport (in
the case of a print-preview, for example). Authors may wish to specify
"fixed" in a media-dependent way. For instance, an author may want an area
to remain at the top of the viewport on the screen, but not at the top of
each printed page.

It would seem that we would want to specify 'fixed' to have the desired
semantics of specifying a anonymously generated region (due to presence of
tts:origin or tts:extent on a content element) in reference to the
viewport, i.e., screen.

[1] http://www.w3.org/TR/2006/REC-xsl11-20061205/#absolute-position