vrend: Add comments to some obscure operations on vrend_renderer

vrend_sampler_view_reference, vrend_shader_state_reference and vrend_resource_reference have some obscure address calculation that cause confusion for some that don't fully understand what is happening there.

Adds new comments to explain what is happening in these codes and why we aren't really de-referencing any NULL pointer.

Signed-off-by: Igor Torrente igor.torrente@collabora.com