mesa: add renderbuffer Depth field

mesa: add renderbuffer Depth field

With glFramebufferTexture, a renderbuffer may support
all layers of the texture, so we need the depth of the
renderbuffer to check for consistency which is required
for framebuffer completeness.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>