gallium: add start_slot parameter to set_vertex_buffers

This allows updating only a subrange of buffer bindings.

set_vertex_buffers(pipe, start_slot, count, NULL) unbinds buffers in that
range. Binding NULL resources unbinds buffers too (both buffer and user_buffer
must be NULL).

The meta ops are adapted to only save, change, and restore the single slot
they use. The cso_context can save and restore only one vertex buffer slot.
The clients can query which one it is using cso_get_aux_vertex_buffer_slot.
It's currently set to 0. (the Draw module breaks if it's set to non-zero)

It should decrease the CPU overhead when using a lot of meta ops, but
the drivers must be able to treat each vertex buffer slot as a separate
state (only r600g does so at the moment).

I can imagine this also being useful for optimizing some OpenGL use cases.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/drivers/rbug/rbug_context.c

@@ -783,7 +783,7 @@ rbug_set_fragment_sampler_views(struct pipe_context *_pipe,
 
 static void
 rbug_set_vertex_buffers(struct pipe_context *_pipe,
-                        unsigned num_buffers,
+                        unsigned start_slot, unsigned num_buffers,
                         const struct pipe_vertex_buffer *_buffers)
 {
    struct rbug_context *rb_pipe = rbug_context(_pipe);
@@ -801,7 +801,7 @@ rbug_set_vertex_buffers(struct pipe_context *_pipe,
       buffers = unwrapped_buffers;
    }
 
-   pipe->set_vertex_buffers(pipe,
+   pipe->set_vertex_buffers(pipe, start_slot,
                             num_buffers,
                             buffers);