fabric.Line._setWidthHeight was assigning left and top as if both
origins were 'center'.
It now uses private helper methods to calculate the distances from
left and top edges of canvas to the line origins.
The data for existing Line.toObject test is updated with origin-relative
coordinates.
Rendering of line inside a path-group was assuming a 'center' origin
for both coordinates.
Context translation done before rendering for lines inside path-groups
now uses private helper methods to calculate distances from the center
of path-group to center of line.
