mesa,glsl,gallium: add GL_OVR_multiview and multiview2

We have had this code for a while, and we had it working with radeonsi at some point, so it should be in a decent shape.

I'm just sharing it in case somebody is interested in finishing it and adding support for their driver. I'm not planning to work on this.

I wonder whether the pipe_framebuffer_state changes are necessary. Lavapipe hasn't needed them for VK_KHR_multiview, so I guess they aren't.