The newly supported extension within Mesa Git is GL_ARB_viewport_array. The GL_ARB_viewport_array extension allows exposing multiple viewports. The OpenGL.org registry specification explains, "This extension enhances OpenGL by providing a mechanism to expose multiple viewports. Each viewport is specified as a rectangle. The destination viewport may be selected per-primitive by the geometry shader. This allows the Geometry Shader to produce different versions of primitives destined for separate viewport rectangles on the same surface. Additionally, when combined with multiple framebuffer attachments, it allows a different viewport rectangle to be selected for each. This extension also exposes a separate scissor rectangle for each viewport. Finally, the viewport bounds are now floating point quantities allowing fractional pixel offsets to be applied during the viewport transform."
Last month ARB_viewport_array was enabled for core Mesa and Intel's driver but now it's been extended to work with the Mesa Gallium3D state tracker and then the R600 hardware driver is leveraging this new code. David Airlie added the Mesa state tracker support and then added the support to R600g. Support for GL_ARB_viewport_array in the RadeonSI and Nouveau drivers will surely come in due time.