Fix connections in heart drawing

[donalgrant]

The connections between the spiro_arc, roll, and rotation pieces don't work. Probably requires:

• Setting guards for the outside loop -- guard will have to be asymmetric -- different for start and finish
• Getting the correct rotation calculation for the inside loop

[donalgrant]

Start by fixing simpler connections:

1. spiro_line works, so try a version of spiro_line which is other than horizontal: call it "spiro_angle" and give it a slope.
2. connect across the ends of spiro_arc.

[donalgrant]

modified spiro_line to take an orientation -- this works and connects. (Should really include an origin location for that function...)

[donalgrant]

spiro_arc is now connected. Almost there with heart drawing: what's tricky is the start and end "guards" for the inside of angles which are drawn by arcs. Guards are working on inside of straight angles drawn by roll function.

[donalgrant]

fixed for poly_heart. Still need to get continuity across roll/arc join.

[donalgrant]

Required new routines to calculated guard for converging arcs, but mission accomplished!