intel: Add fields to intel_renderbuffer for unwrapping packed depth/stencil buffers

intel: Add fields to intel_renderbuffer for unwrapping packed depth/stencil buffers

Add the following fields:
    intel_renderbuffer.wrapped_depth;
    intel_renderbuffer.wrapped_stencil

If the intel_context is using separate stencil and the renderbuffer has
a packed depth/stencil format, then wrapped_depth and wrapped_stencil are
the real renderbuffers.

Alter the following functions to accomodate the wrapped buffers:
    intel_delete_renderbuffer
    intel_draw_buffer
    intel_get_renderbuffer
    intel_renderbuffer_map
    intel_renderbuffer_unmap

Subsequent commits allocate renderbuffer storage for wrapped_depth and
wrapped_stencil.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chad Versace <chad@chad-versace.us>