The shortest path query

This is the fundamental pathfinding query.
This query returns the shortest set of movement sections taking an agent from a start position to a specified target position, if one exists.

The query is provided as iAgent::findShortestPathTo() and as iMesh::findShortestPath().
The iMesh version doesn't require an agent.
The iAgent version finds the shortest path from an agent's current position to a specified target.

Calling the query is pretty simple:

if(testbed->getLeftMouseState())
{
// path towards position under mouse
	path = nullptr;
	cPosition target=testbed->positionAtMouse();
	if(target.cell!=-1)
		path = agent->findShortestPathTo(context, target);
}

Note that passing in our collision context means that the query will automatically take account of any obstacles added to that context.