Keep motion EndTime absolute and rescale correctly on speed change

[FMsongX2]

Problem

CubismMotionLayer.SetStateSpeed (reached via the public CubismMotionController.SetAnimationSpeed) writes a relative value into EndTime, which is an absolute timestamp everywhere else — set as StartTime + length / speed on play and read via Time.time > EndTime. The relative value lands in the past, so Time.time > EndTime is immediately true: the motion fires its end event at once, and its fade-out weight collapses to 0 (the now-negative EndTime - time is clamped by GetEasingSine), cutting the motion instead of playing it out. It triggers on any speed change, including speed == 1.

Reproduction

Play a non-looping motion through CubismMotionController, then call SetAnimationSpeed(0, index, 1.0f) (speed unchanged — a no-op):

playing: EndTime = 142.4 (absolute), Time.time = 136.8, ~5.6s remaining
after SetAnimationSpeed(1.0):  EndTime = 142.4 -> 5.57
  Time.time(136.8) > EndTime(5.57)       -> end event fires immediately
  fade-out weight: (5.57 - 136.8) < 0    -> GetEasingSine clamps to 0 (motion cut)
next Update():  IsPlayingAnimation -> false   (a motion with 5.6s left is cut instantly)

The API is public but no sample calls it, which is why this went unnoticed.

Fix

Rebuild EndTime from the remaining content time, keeping it absolute:

EndTime = Time.time + (EndTime - Time.time) * previousSpeed / speed;

previousSpeed is the value being overwritten, so the rescale is correct for any prior speed, not only 1 — (EndTime - Time.time) is the wall-clock remaining under the old speed, * previousSpeed gives the remaining content time, / speed converts it to the new speed. The EndTime == -1 "no end" sentinel (MotionLength <= 0) is left untouched, and the rescale is skipped when the previous speed was 0 so speed == 0 (pause) paths stay unchanged instead of producing NaN.