meta: leaking of BO with DrawPixels

ctx->Unpack.BufferObj wasn't unreferenced.

Fixes: d492e7b017 (meta: Fix invalid PBO access from DrawPixels when
trying to just alloc.)
CC: Eric Anholt <eric@anholt.net>
Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/common/meta.c

@@ -1468,6 +1468,8 @@ _mesa_meta_setup_drawpix_texture(struct gl_context *ctx,
          /* load image */
          _mesa_TexSubImage2D(tex->Target, 0,
                              0, 0, width, height, format, type, pixels);
+
+         _mesa_reference_buffer_object(ctx, &save_unpack_obj, NULL);
       }
    }
    else {