vallium: initial import of vulkan/gallium frontend for software drivers.

This is the initial import of the vulkan/gallium frontend. It only works with llvmpipe.

There are some setup patches for add a memory allocation API. Push constant support for llvmpipe (so gears demo runs) ZS helpers WSI software path support Vallium frontend.

This is pretty incomplete from the driver side, and lots of improvement on the driver side are needed before this will be considered compliant, but the frontend is mostly good enough now.