 true if two passed bodies are colliding.
The approach is to test for five situations.  If any are true,
the bodies are definitely not colliding.  If none of them
are true, the bodies are colliding.</p>
<ol>
<li>b1 is the same body as b2.</li>
<li>Right of <code>b1</code> is to the left of the left of <code>b2</code>.</li>
<li>Bottom of <code>b1</code> is above the top of <code>b2</code>.</li>
<li>Left of <code>b1</code> is to the right of the right of <code>b2</code>.</li>
<li>Top of <code>b1</code> is bel