The latest extension to talk about is ARB_framebuffer_no_attachments. The ARB_framebuffer_no_attachments extension is mandated by OpenGL 4.3 and already supported by core Mesa and the i965 Intel driver.
With 17 patches published on Friday, this extension would be wired in for the Gallium3D drivers including R600g and RadeonSI while there is also work pending on Nouveau NVC0.
Once ARB_framebuffer_no_attachments is in Mesa Git, there's just three extensions each for RadeonSI and Nouveau NVC0 before the GL 4.3 requirements are met (plus the one each for GL 4.2). This year continues to look bright for advancing OpenGL 4 support within Mesa for open-source graphics drivers.