The same logic from D17025 is used in other places in the curve code. This patch uses the class for the evaluated point offsets and the Bezier control point offsets. This helps to standardize the behavior and make it easier to read. Previously the Bezier control point offsets used a slightly different standard where the first point was the first offset, just so they could have the same size as the number of points. However two nodes used a helper function to use the same `OffsetIndices` system, so switch to that there too. That requires removing the subtraction by one to find the actual offset. Also add const when accessing data arrays from curves, for consistency. Differential Revision: https://developer.blender.org/D17038