mesa: Save and restore GL_RASTERIZER_DISCARD state during meta ops.

mesa: Save and restore GL_RASTERIZER_DISCARD state during meta ops.

During meta-operations (such as _mesa_meta_GenerateMipmap()), we need
to be able to draw even if GL_RASTERIZER_DISCARD is enabled.  This
patch causes _mesa_meta_begin() to save the state of
GL_RASTERIZER_DISCARD and disable it (so that drawing can be done
during the meta-op), and causes _mesa_meta_end() to restore it.

Fixes piglit test "EXT_transform_feedback/generatemipmap discard" on
i965 Gen6.

Reviewed-by: Brian Paul <brianp@vmare.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>