ExpressionEngine: Length2D() calculation is not accurate for arcs

[danstoica]

The diffence came from the fact the arcs are tessellated and the length was computed by adding up the length of individual segments. The reason doing so is that the functionality to compute the radius of the circle and the orientation of the arc is not trivial and it lives in FdoSpatialUtilityCircularArc which is not exported.

Another approach is to avoid code duplication and compute the radius and the angle by using basic trigonometry on the circle. That is, the algorithm needs 2 equal adjacent segments on the circle which are provided by tessellation.