The extension that's now flipped on for Intel Ivy Bridge "Gen7" hardware and newer is the GL_AMD_vertex_shader_layer extension.
From the OpenGL.org registry specification, "This extension exposes the gl_Layer built-in variable in the vertex shader, allowing rendering to be directed to layered framebuffer attachments with only a vertex and fragment shader present. Combined with features such as instancing, or static vertex attributes and so on, this allows a wide variety of techniques to be implemented without the requirement for a geometry shader to be present."
Patches for implementing the GL_AMD_vertex_shader_layer OpenGL extension within Mesa have been floating around on the developers' patch list for months but now as of this weekend it's finally been fully wired-up and enabled for Intel on Gen7+ graphics hardware. The final patch enabling the support for this GL extension can be found by this Git commit.