Some Mesa developers have already started tackling some of the easier OpenGL 4.5 extensions and today another can be crossed off the list. Thanks to Tobias Klausmann. GL_ARB_conditional_render_inverted is now supported by Mesa. The core work for GL_ARB_conditional_render_inverted is complete and is implemented currently by the Gallium3D-based Nouveau NVC0 (Fermi+), Softpipe, and LLVMpipe drivers. Support will surely come in time for mainline Mesa with this extension for the RadeonSI Gallium3D and Intel drivers.
The ARB_conditional_render_inverted is described by the OpenGL.org registry as simply an extension that "adds new modes to BeginConditionalRender which invert the condition used to determine whether to draw or not."
This work will be found as part of the upcoming Mesa 10.3 release, which might be known as Mesa 11.0 should the GL4.0 compliance be reached in the very near term.