Emmanuele Bassi is the developer that for over one year has been working towards a canvas API inside GTK. It's also worth noting that Emmanuele Bassi is also the maintainer of the Clutter tool-kit, with some of his and others agenda being to move more of Clutter into GTK. Instead of a Clutter 2.0 release, there's hope for merging a subset of Clutter's features directly into GTK+.
Among the reasons for wanting to have a scene graph within GTK+ itself is for handling widgets in more complex states, better supporting CSS styling of widgets, etc. Emmanuele's presentation at this year's GUADEC covers his issues in OpenGL, Cairo, potential issues with GTK's HTML5 Broadway back-end, his Graphene project, etc.
Emmanuele Bassi expects to have the core API and implementation of the GTK+ Scene Graph Kit sorted out in time for the start of GNOME 3.15 development cycle, which means it would start reaching user's Linux systems in early 2015 with the GNOME 3.16.0 stable release.
You can learn more about this work on the GTK+ Scene Graph Kit (GSK for short) via Bassi's blog post.