Google engineers working on Chromium OS, their Linux-based browser operating system, previously was working on the unofficial Intel Gallium3D driver to enhance the "i915g" support for their Chromebooks. They've also made some other improvements to Mesa, but now they've made this interesting commit to the common code-base for implementing GLX_EXT_texture_from_pixmap support in software for Gallium3D. The patches were sent in by Stéphane Marchesin, the founder of the Nouveau driver project that later joined Google's Chromium OS team, and was signed off by several other Chromium developers.
GLX_EXT_texture_from_pixmap is the GLX extension that several years ago was sought after by Linux desktop enthusiasts in the early days of AIGLX and with the Compiz/Beryl compositors as being the critical extension to support composited window management at the time. This extension allows a color buffer to be used for both rendering and texturing, per the specification.
Bringing this to the software on Mesa Git is done by three patches amounting to about 200 lines of new code and replicating one file for the DRI state tracker. The patch series starts here.
With this support it may now be possible (I am about to test the code and haven't heard reports from anyone else yet) to run the GNOME Shell, Compiz, or other compositing window managers on a Gallium3D software driver off the CPU. It's also a question of how's the performance and whether there are any other rendering issues when using such drivers.
It was last year that David Airlie was playing around with GLX_EXT_texture_from_pixmap support in swrast, the classic Mesa software rasterizer, but he didn't do much more in this area after that point. Right now the only major distribution to be using the LLVMpipe software driver as its software fall-back when no GPU hardware driver is available is Fedora 15 and later.