Added support for smoothing endpoints of animation clips

Added "loopBlendTime" property for animation clips to support interpolating between the end points of an animation clip when looping. This allows clips to animate smoother when the end points of a clip do not exactly match up (which is all too common with animated character content).

A new overloaded version of Curve::evaluate was also added to support this feature.

Fixed some subtle bugs in both gamepaly and gameplay-encoder (FBX) related to slightly incorrect animation frame timings.