Skeletal models functionality: multianimation

[stohrendorf]

• add multianimation system (animation layers): weapon animations (not a car / bike case though; it's two entities with the same coordinates / orientation);
• add head, torso rotation for actor->look_at and actor->targeting (additional and final multianimation);
• add proper meshswap system (i.e. Lara's head lipsync, etc.), which in turn requires mesh indexing abstraction layer.
• fix animation interpolation in animation switch case (e.g. buttons) - possibly just bypass slerp for problematic ones.;

[TeslaRus]

animation layers Implemented, but it is necessary to add simple functional interface, instead direct access to structure flags and fields;