Add <RubbleActor> tag in <VisualActor> Element for Building Templates

[michael]

It would be nice to have a <RubbleActor> tag added for the <VisualActor> element in building entity templates, like so:

  <VisualActor>
    <FoundationActor>structures/fndn_2x2.xml</FoundationActor>
    <Actor>props/special/palisade_rocks_outpost.xml</Actor>
    <RubbleActor>structures/rubble_2x2.xml</RubbleActor>
  </VisualActor>

The specified Rubble actor would appear on the ground when a building dies and collapses. It would then "decay" sink or fade at some specified rate. Perhaps with another new element, like:

  <RubbleDecay>
    <DelayTime>30.0</DelayTime>
    <SinkRate>0.5</SinkRate>
    <SinkAccel>0.5</SinkAccel>
  </RubbleDecay>

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This ticket has been split off from ticket #131.