gallium, llvmpipe, lavapipe: KHR_multiview support

This adds a pretty rudiementary implementation of KHR_multiview to lavapipe.

It makes on change to the gallium draw api command which might be up for debate, otherwise there is a bunch of code inside draw/llvmpipe to add support to the shaders and to pass the draw index between draw/llvmpipe in the right place.

It also adds lavapipe support for clearing and drawing.

This passes all the current CTS tests and renders the multiview demo.