Beizer curve interpolation sometimes incorrect for single frames

[ivelieu]

For specific parameter curves for the keyframe editor, the beizer curve calculation ends up being incorrect for specific frames.
The keyframe editor by default uses spline smoothing, which is probably why this was not encountered by others.

Demonstration:
If you import the following "raw data" and set it to a beizer curve over a 120 frame video:

1 -0.7
15 -0.63168449
27 1.00066845
68 0.79545455
90 -0.7
120 -0.7

frame 47 will have a parameter value of 47.5 instead of the expected ~0.89.
This is what the curve looks like in the keyframe editor:

Here is a video of it jumping:
https://github.com/thargor6/JWildfire/assets/28797826/fcb214b1-a003-467e-9bc0-07d0bcfbb0a7