Fixed start time when normalized time is not zero

[faveris]

In our game we have a character and a weapon as separate meshes. We use MeshAnimation for character and simple legacy animation for weapon. It works fine, but when start time is not equal to zero, animations go out of sync:

bug.mov

Example code for reproduction:

// Mesh animation
_meshAnimator.Play(clipName, 1.5f, 0.3f);

// Legacy animation
AnimationState state = _animation[clipName];
state.speed = 1.5f;
state.normalizedTime = 0.3f;
_animation.Play(clipName);

I found two issues which lead to this bug in MeshAnimationAssetExtensions.Play method, when calculating start time:

1. We should decrease start time but not increase, because it should be considered that animation is kind of started in the past when normalized time is greater than zero.
2. We have to take into account that speed could be not equal to 1, therefore data.lengthSeconds should be devided by speed (or better use already calculated s)

Result after a fix:

bugfix.mov