Re: [svg][filter-effects][css-masking] objectBoundingBox/userSpaceOnUse in HTML

Note the closely-related question of what to do for paint servers in a
CSS context, which Images defines in line with Firefox implementation,
based on an old blog post of roc's:
<http://dev.w3.org/csswg/css-images/#paint-sources>

There, the space you're drawing into is identical for both cases; the
only difference is the size of a coordinate unit.

I think the same really makes sense here, for things like clip-path -
use the bounding rect.  If we have a way to manipulate the box used,
it should also affect stuff using objectBoundingBox, in the same way
as images.

~TJ