fix traversal, improve search region.
fix traversal, improve search region.

Two bug fixes:
1. Before we would return the current node if it didn't have a
matching child. Now, we are just going to return `null`.
2. Before, as long as you clicked within a node's range, you would
"click" on a node. What we really want is to return a node iff you
click on the node circle (within the radius.)

TheIronDeveloper/dom-to-canvas