This commit implements generic evaluation for Bezier curves (which is
really just linear interpolation, since attributes are not stored on
Bezier handles). For complete parity with the old curve type, we would
have to add options for this (RNA: `Spline.radius_interpolation`),
but it's not clear that we want to do that.
This also adds a generic `interpolate_to_evaluate` utility on curves
that hides the implementation details. Though there is theoretically
a performance cost to that, without some abstraction calling code
would usually be too complex.
Differential Revision: https://developer.blender.org/D14447
cea51c1bb5