draw/llvmpipe: allow independent so attachments to the vs

draw/llvmpipe: allow independent so attachments to the vs

When geometry shaders are present, one needs to be able to create
an empty geometry shader with stream output that needs to be
resolved later and attached to the currently bound vertex shader.
Lets add support for it to llvmpipe and draw. draw allows attaching
independent stream output info to any vertex shader and llvmpipe
resolves at draw time which vertex shader the given empty geometry
shader should be linked to.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>