[css-filters] enable-background in an HTML context

All,

the current filters spec (found here:
https://dvcs.w3.org/hg/FXTF/raw-file/tip/filters/index.html) specifies how
the background image is created in SVG, but it doesn't say how this is done
when a filter is applied to HTML content.
David Baron said that the CSS painting order (which can be found here:
http://www.w3.org/TR/CSS2/zindex.html) should probably define what this
background is.

I think that we can reuse the current definition of 'enable-background' but
for HTML, we should state that it is calculated after the painting order is
established.

Rik