gallium: rename pipe_buffer_handle to pipe_buffer, rework pipebuffer/ code

Provide an actual definition of the pipe_buffer struct,  containing
the parameters used to create the buffer, and its refcount.

Shift refcounting buffers out of the winsys interface, similar to
surfaces & textures.

Rework pipebuffer/ to reflect the fact these changes, and also Michel's
reworking of the buffer interface.

src/mesa/state_tracker/st_context.c

@@ -52,6 +52,7 @@
 #include "st_program.h"
 #include "pipe/p_context.h"
 #include "pipe/p_winsys.h"
+#include "pipe/p_inlines.h"
 #include "pipe/draw/draw_context.h"
 #include "pipe/cso_cache/cso_cache.h"
 
@@ -152,7 +153,7 @@ static void st_destroy_context_priv( struct st_context *st )
 
    for (i = 0; i < Elements(st->state.constants); i++) {
       if (st->state.constants[i].buffer) {
-         ws->buffer_reference(ws, &st->state.constants[i].buffer, NULL);
+         pipe_buffer_reference(ws, &st->state.constants[i].buffer, NULL);
       }
    }