path_arc_3p - 3-point arcs

`local arc_3p = require'path_arc_3p'`

Math for 2D 3-point circular arcs defined as:

x1, y1, xp, yp, x2, y2

where (x1, y1), (xp, yp) and (x2, y2) are the arc's endpoints.

arc_3p.to_arc(x1, y1, xp, yp, x2, y2) ->
   cx, cy, rx, ry, start_angle, sweep_angle, rotation, x2, y2

Convert a 3-point arc an elliptic arc.

If the endpoints conicide are collinear then the parametrization is invalid and nothing is returned.

arc_3p.split(t, x1, y1, xp, yp, x2, y2) ->
   x11, y11, x1p, y1p, x22, y22
   x21, y21, x2p, y2p, x22, y22

Split a 3-point arc at time t into two arcs.