Add a new define (USE_SIMD16_VS), to denote calling a 16-wide vertex shader.
This is needed because the mesa driver can do 16-wide shaders, but rasty
cannot yet, so we need to distinguish.

Create a new VertexID entry (VertexID16) for the USE_SIMD16_VS case, since
we need to format the vertex id in a way that is digestible by the 16-wide VS

Disabled for now.  To be enabled in a future checkin when driver work
is complete.

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>