Re-organisation: <diffuse>, <ambient>, <emission>, <specular> are
now groups with members <red>, <green>, <blue>, <factor>, <offset>,
and their <*-prop> forms. Additionally, there's an option <property-base>
that can be used to set a path that is prepended to all <*-prop> paths.
It defaults to an empty string. Rationale: see model-howto.html.
here is the promised material animation. It looks a bit longish, but that
wasn't avoidable -- there are simply too many parameters to consider. I tried
hard, though, to make the animation fast by only doing the necessary stuff.
It doesn't affect the frame rate here with my test model. The animation is
heavily based on Jim's "material-emission" animation.
* implementation of the "material" animation (this required to make the
texture path available) + documentation update ($FG_ROOT/Docs/)
* fix some more return values (texture animations, and select) for the
shadow problem (and some in anticipation of other problems :-)
* fix compiler warning
