Aternative to #1777.  Properly closes the paths returned by clip_to_rect and adds a new clip_to_bbox method to Path that returns a Path object.  Adds a test that tests clipping compound paths and multiple subsegments.
Use `np.empty` for a small speed improvement.
Document that clip_to_bbox requires closed paths.
Merge pull request #1778 from mdboom/clip_path_to_rect

Fix clip_path_to_rect, add convenience method on Path object for it