Improve Spline/Bézier Support

LibreCAD has basic support for B-spline and quadratic splines, but many interesting features are still missing. For example, methods for trimming, offsetting, hatching, tangential directions, etc. are required to allow extensive usage of splines in LibreCAD. A rather comprehensive collection of spline mathematics can be found at A Primer on Bézier Curves Examples of spline features to implement: