Spline gradients not tessellated properly

[space928]

Describe the bug

The spline tessellation algorithm doesn't take into account the extra geometry needed to represent gradient changes (honestly I'm not even sure if gradients works correctly at all).

To Reproduce

Import any non curved spline which includes gradient changes

Expected behaviour

Geometry should be created as needed to represent the gradient

Additional context

This should just be a case of modifying the tessellation increment calculation to consider the gradient change: https://github.com/space928/Blender-O3D-IO-Public/blob/dd30231969ecf0acde33fa9f8598e22e364e5daa/o3d_io/io_omsi_spline.py#L67-L85