Move frame interval from the Composition to the frame

[xorgy]

Real displays don't follow exact fraction-of-a-second frame rates, and animations are rarely authored with continuous frames on a fixed interval anyhow (whether they are tweened or stop-motion), so the correct place for frame intervals is on the frames/tweens, and not the composition as a whole. Re-pacing animations is a separate concern.

[xorgy]

There are design considerations for this. To avoid seams between synchronized animations, and edge cases where a frame is rendered without a layer that is expected to be there, but is not due to different rounding: it is still desirable to have integer times at the frame and layer level, so a Composition-level (integer fraction) time base may still be there.