[css3-background] Corner clipping for images and gradients

In Mozilla bug 969263[1] the question came up whether to clip gradients,
elements and images defined in border-image. The current WD says[2] this:

"A box's backgrounds, but not its border-image, are clipped to the
appropriate curve (as determined by ‘background-clip’)."

According to David Baron the WG's intent for url() is when defined in
border-image it is allowed to *overhang* the box.
Though this may not be what the author expects, especially in cases where
gradients or elements are used. See the test case on the report mentioned
above.

One idea I can come up with to solve this is to add a new property
‘border-clip’, which accepts the same values as ‘background-clip’ plus an
additional value, which avoids clipping, e.g. ‘none’.

Sebastian

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=969263
[2] http://www.w3.org/TR/css3-background/#corner-clipping