Improved AnimCurve Tangents

[david-cattermole]

Sometimes the tangents look very strange and unnatural (to what an animator would like). The curve tangents are wavey, users would expect the tangents to be similar to the source.

The computed keyframe tangents are not taking the slope (tangent) of the source curve into account when solving the new tangents.

Example error: The selected curve is the sparse destination curve and it has wobbly tangents, where it should be smooth.