Mesa Lands Support For GL_EXT_window_rectangles
GL_EXT_window_rectangles is a newer OpenGL extension and explained via the OpenGL.org registry, "this extension provides additional orthogonally aligned 'window rectangles' specified in window-space coordinates that restrict rasterization of all primitive types (geometry, images, paths) and framebuffer clears."
Ilia Mirkin did the plumbing work for the GL_EXT_window_rectangles extension in Mesa and Gallium3D's Mesa state tracker. So far he's proceeded to hook up this extension for Nouveau's NV50 and NVC0 extensions.
The Mesa work on GL_EXT_window_rectangles was mainlined on Saturday and can be found via Mesa Git master until the September stable release update.