Add support for delta-time playback

[ebeneliason]

This adds the option to use delta-time playback as needed to perform time-based synchronization or achieve time dilation effects. Calling update(dt) each frame will override the underlying animation loop timing and set the appropriate frame according to the elapsed time, while respecting all available options (delay, loop, step, startFrame, endFrame, paused, etc.). If update is not called, the behavior remains unchanged.