return a point at the given position.

support pointOnPath() and layer/group child exploration

{pathProperty}.pointOnPath(percentage = 0.5, t = time)

get the x,y coordinates of an arbitrary point along a path.
the point is expressed as a percentage of the arc-length of the path.
0% is the first point and 100% is the last point.

when the path is closed, 0% and 100% will return the same coordinates.
percentage of arc-length is used to ensure uniform speed along the path.
other than 0% and 100%, percentages do not necessarily correlate with
the Bezier points on the path - For a path with three points,
the second point will not necessarily be at 50%.

this also means that for an open path and closed path with identical points,
the percentage along the open path will not return the same coordinates
as the closed path due to the additional length of the closed path.

optionally specify the time at which to sample the path.

issue: #2233