remove the gap from cloth sewing

Problem : when using cloth-sewing edges, blender cloth simulation leaves a gap, and the gap gets bigger if there is any stretching force on the cloth. Increasing max-sewing-force can reduce the gap, but adds so much energy and force to the sewing process it tends to explode and create crazy oscillation.

Solution Change the Cloth Simulation Sewing code to close the gap smoothly and virtually "Weld" the gap endpoints together so they act as one during the duration of the simulation.

Perhaps the weld could happen with a smooth exponentially increasing force, and then weld the endpoints together so the two points are treated as one by the simulation.

201904-cloth-model-test.blend

Why does the gap happen : Blender's cloth proportionally distributes stretch forces around all springs under tension -- including the sewing springs. This means the sewing gap always gets some amount of the force distributed to it. Because the sewing-force is finite, this always results in a gap, linearly proportional to the amount of force distributed to the sewing spring.

Why doesn't max-sewing-force fix this : The "max sewing force" parameter isn't very usable in it's current state, because the larger you make it, the faster the sewing happens, which adds more energy to the fabric during the sewing, which oscillates around the fabric causing it to expand and wobble, and sometimes explode.