gallium: Unify reference counting.

The core reference counting code is centralized in p_refcnt.h.

This has some consequences related to struct pipe_buffer:

* The screen member of struct pipe_buffer must be initialized, or
  pipe_buffer_reference() will crash trying to destroy a buffer with reference
  count 0. u_simple_screen takes care of this, but I may have missed some of
  the drivers not using it.
* Except for rare exceptions deep in winsys code, buffers must always be
  allocated via pipe_buffer_create() or via screen->*buffer_create() rather
  than via winsys->*buffer_create().

src/gallium/auxiliary/util/u_debug.c

@@ -680,7 +680,7 @@ void debug_dump_surface(const char *prefix,
    
    screen->transfer_unmap(screen, transfer);
 error:
-   screen->tex_transfer_release(screen, &transfer);
+   screen->tex_transfer_destroy(transfer);
 }
 
 
@@ -785,7 +785,7 @@ debug_dump_surface_bmp(const char *filename,
       }  
    }
 
-   screen->tex_transfer_release(screen, &transfer);
+   screen->tex_transfer_destroy(transfer);
    
    util_stream_close(stream);
 error2: