When the background-clip of the background is smaller than the
background-clip of blended images, not pushing a group is wrong.
Test testing exactly that included.
CSS supports blend modes, in which a series of layers are
merged together according to the given operation or set of
operations.
Support for blend modes landed on Cairo, which exposes all
the commons and also the exquisites blend modes available.
Adding support for blend modes, then, is just a matter of
using the available Cairo operations.
This patch adds the background-blend-mode CSS enum property,
and adapts the background rendering code to blend the backgrounds
using the available blend modes when they're set.
https://bugzilla.gnome.org/show_bug.cgi?id=768305
