The GNOME 3.13.90 Beta release is scheduled to happen today and as such the Mutter and GNOME Shell updates were checked in this week. With the Mutter 3.13.90 comes an enforcement that XSync() is only ever called once per-frame, the GLSL support is optional, gesture and touch events are now handled on Wayland, and there's a variety of other fixes/changes. The Mutter 3.13.90 changes can be found via its release announcement.
The GNOME Shell 3.13.90 update makes the use of GLSL optional, improves window manager animations, handles touch events, and has too a wide variety of other changes. GNOME Shell 3.13.90 details can be found via this mailing list post.
Mutter and GNOME Shell making the GL Shading Language (GLSL) optional can be traced to this bug report with up to now some GLSL being used unconditionally -- even for hardware not supporting the OpenGL shaders -- which would lead to hardware crashes when these window effects were encountered.
The handling of gesture and touch events on Wayland is another great addition that's good to see ahead of the GNOME 3.14 release. The Wayland touch/gesture details can be read about via this GNOME bug report.