add CAG (Constructive Area Geometry) support

[pixelzoom]

This could be used immediately in graphing-lines.

[jonathanolson]

See details in https://github.com/phetsims/kite/issues/20

Also necessary for more advanced BAM 3D edge cases, and for WebGL work.

[pixelzoom]

Another request for this feature. In ph-scale, I have all kinds of odd shapes that I need to draw. They would be trivial with CAG. They are going to take more time without CAG, and will require copying more code from kite (eg, rounded corners).

[pixelzoom]

Another need for this feature today, in sun.ComboBox. Request to add a rectangle behind the arrow button, which would have square corners on left end, round corners on right end.

[pixelzoom]

Another need for this feature today in pH Scale, yet another rounded rectangle combined with another shape. I'm getting good at using Shape.arc.

[pixelzoom]

Removing the "high priority" label, because it's not.

[jessegreenberg]

This would be very helpful in Capacitor Lab, shapes in that sim were mostly put together using CAG. It is not critical though, I can still redraw shapes using arcs and z layer ordering.

[jonathanolson]

I've implemented the large bulk of cases for CAG (referenced some commits towards https://github.com/phetsims/kite/issues/20). shapeUnion, shapeDifference, shapeIntersection and shapeXor (methods on Shape) are implemented. getArea and similar functions came for free.

Some outlines of the general CAG process exist in https://github.com/phetsims/kite/issues/20, but this issue is more suited towards the subset that is actually completed and should be reviewed.

For review, I'd recommend:

• All of kite/js/ops (Boundary, BoundsIntersection, Edge, Face, Graph, HalfEdge, Loop, Vertex). Main logic in Graph and BoundsIntersection (with associated helper object types).
• For each segment type (Arc/Cubic/EllipticalArc/Line/Quadratic), the getOverlaps method on the type (including any associated helper methods) and getSignedAreaFragment segment method.
• Cubic.getSelfIntersection

Definitely let me know where documentation can be improved (anything that's confusing). Thanks!

Assigned to @jessegreenberg for review.

[jonathanolson]

I will need to implement the curvature-based discrimination of half-edges when sorting incident edges to vertices. CLB has a case with a line and arc with the same tangent.

[jonathanolson]

Implemented the curvature-based method described above.

Additional things added (which could benefit from review):

• Arc.intersect (arc-arc intersection)
• Arc.getCircleIntersectionPoint (circle-circle intersection)
• Shape/Subpath/Segment serialization/deserialization (Supports shape.serialize() to JSON, then Shape.deserialize( json ) to create an equivalent shape with exact numerical precision).
• static Shape.union, Shape.intersection and Shape.xor that apply to an array of shapes.
• Added RayIntersection and SegmentIntersection types (to get rid of duck-typed intersection results)
• Computation of parametric values for every segment.intersection( ray ).
• Line.intersectOther (customized higher-performance intersection with line segments and any other type of segment)
• Line.intersect (line segment - line segment intersection)

Extra credit: Try creating two shapes whose union either isn't correct, or asserts out!