Line coordinates are correct

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.
Fix group mocks in Line tests

Added missing semicolons in tests.

Refactored Line centerToCenterGetter to improve readability.
Removed Line._getCenterToCenter helpers

Pre-rendering context translation for lines in path-groups uses the
pre-existing getCenterPoint.

Removed tests for removed code.
Merge pull request #1143 from maciej-filip-sz/bug/Line-coords-rebased

Line object coordinates are correct