[KeyFrame] skipping frames with wrong time

+ sometimes in scml file there are frames that are pointing to a
different time. these frames are skipped from the animations

! this created a bug with objects disappearing, these frames have to be
added it seems, but only for “show” attribute
[Animations] fix for a bug intruduced in previous commit

+ all spriter nodes got “active” variable, telling whether they should
be taken into account when calculating changes or just as markers of
“being shown”
+ keyframer sets this new activity parameter
+ prefab maker propagates this into unity game object
+ animation clip skips game objects that don’t want to be used in
calculations, but preserves them for visibility