The issue was caused by handles not being written to the new mask
spline shape: it was always written as (0, 0), which was making the
handle calculation go wrong later on.
Solved by allocating a temporary array of bezier points and calculating
handles for them. While this is an extra array allocation it is only
done for a small amount of points and it is not in the critical code
path. Having this as an extra array helps to overcome a limitation of
the current mask shape API.
Additionally, disable the interpolation for the shape change:
the spline is brand new, there is nothing to be interpolated there.
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