First attempt at building vertex buffers post-clip.

First attempt at building vertex buffers post-clip.

Build a buffer of contigous vertices and indices at the backend of our
software transformation/clipping path.  This will become the mechanism
for emitting buffers of vertices to rasterization hardware.

This is similar to but not the same as the post-transform vertex cache.
In particular, these vertices are subject to clipping, culling, poly offset,
etc.  The vertices emitted will all be used by hardware.

TODOs include the actual transformation to hardware vertex formats, moving
this out of softpipe to somewhere more useful and allowing >1 primitive to
share the generated VB.