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>
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