Fix aligned box collision

[erincatto]

The boxes are slightly separated and clipping is just giving one contact point.

https://discord.com/channels/460295137705197568/1064661081903075429/1330362522888437814

	explicit SingleBox( Settings& settings )
		: Sample( settings )
	{
		if ( settings.restart == false )
		{
			g_camera.m_center = { 0.0f, 2.5f };
			g_camera.m_zoom = 3.5f;
		}

		float extent = 0.5f;

		{
			b2BodyDef bodyDef = b2DefaultBodyDef();
			b2Polygon box = b2MakeBox( extent, extent );
			bodyDef.position = { 0.0f, 20.0f };
			;
			bodyDef.type = b2_dynamicBody;
			b2BodyId bodyId = b2CreateBody( m_worldId, &bodyDef );
			b2ShapeDef shapeDef = b2DefaultShapeDef();
			b2CreatePolygonShape( bodyId, &shapeDef, &box );
		}
		{
			b2BodyDef bodyDef = b2DefaultBodyDef();
			//b2Polygon box = b2MakeSquare( 0.9f * extent );
			b2Polygon box = b2MakeSquare( extent );
			bodyDef.position = { 0.0f, 0.0f };
			bodyDef.type = b2_staticBody;
			b2BodyId bodyId = b2CreateBody( m_worldId, &bodyDef );
			b2ShapeDef shapeDef = b2DefaultShapeDef();
			b2CreatePolygonShape( bodyId, &shapeDef, &box );
		}
	}

[erincatto]

Reproduced this in Manifold sample with square vs square and a separation of 0.01