css: add background-blend-mode support

css: add background-blend-mode support

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