A simple box stack is not stable without sleeping

[abigplatypus]

Hey Randy,

First off, thanks for the code. It's a great learning tool. There's something I wanted to bring to your attention and understand whether it's expected. To reproduce:

1. Clone the latest version of the repo.
2. In Demo.cpp, change bool enableSleep = true; to bool enableSleep = false;
3. In BoxStack.h, change the code that adds the boxes so that it adds a single stack of 5:

for ( i32 i = 0; i < 5; ++i )
{
		        r32 k = 0;
		        r32 j = 0;
			bodyDef.position.Set(  1.0f * j, 1.0f * i + 1.0f,+ 1.0f * k );
			body = scene.CreateBody( bodyDef );
			body->AddBox( boxDef );
}

4. Run the BoxStack demo.

After a few seconds (~5), the box stack collapses. This is without changing anything else and just by disabling sleep. Is this expected? I'd expect a warm-starting solver to be able to resolve a box of 5 boxes, especially at the default 20 iterations, without needing to put the boxes to sleep.

I'd love any insight!