As the global object of a context is rooted, if we want the GC to act on these objects we need to take them out of the globals. https://bugzilla.gnome.org/show_bug.cgi?id=678737
https://bugzilla.gnome.org/show_bug.cgi?id=678737
https://bugzilla.gnome.org/show_bug.cgi?id=672321
Right now we only test the cairo rendering path https://bugzilla.gnome.org/show_bug.cgi?id=672321
Implement the background-size CSS property, specified by the CSS Backgrounds and Borders Module Level 3, including the keywords "contain", "cover", and fixed-size backgrounds. https://bugzilla.gnome.org/show_bug.cgi?id=633462