MP Fallback models

MP Fallback models allow an aircraft developer to set what aircraft model be used to render their aircraft over MP if a user doesn't have their aircraft installed.

To make use of this feature, simply set /sim/model/fallback-model-index in the aircraft -set.xml file to one of the indexes in /AI/Aircraft/fallback_models.xml.

This is then transmitted over the MP system and used by other clients.

For example;

<sim>
  <model>
    <fallback-model-index>6</fallback-model-index>
...

will use the Cub model, as index 6 refers to it in fallback_models.xml:

<PropertyList>

<!-- Default aircraft.  Will also be used if an AI model is not found -->
<model n="0" type="string">/Models/Geometry/glider.ac</model>

<!-- 1-50   Single engine GA aircraft -->
<model n="1" type="string">AI/Aircraft/c172/c-fgfs.xml</model>
<model n="2" type="string">AI/Aircraft/c177/c177-g-brdo.xml</model>
<model n="3" type="string">AI/Aircraft/c182/Models/c182-ai.xml</model>
<model n="4" type="string">AI/Aircraft/c182rg/Models/c182rg-ai.xml</model>
<model n="5" type="string">AI/Aircraft/Cessna208Caravan/Models/Cessna208-ai.xml</model>
<model n="6" type="string">AI/Aircraft/Cub/Models/Cub-ai.xml</model>

...

</PropertyList>