Update accessible name calculation algorithm for img to use figcaption

if an img has no other naming mechanism, and it is a descendent of a figure element with a figcaption, but the figure has no other descendants, then the figcaption can be used as the image's accname.

html-aam/index.html

@@ -16256,13 +16256,14 @@ <h4>`img` Element Accessible Name Computation</h4>
               <a href="" class="accname">Accessible Name and Description: Computation and API Mappings</a>.
             </li>
             <li>
-              Otherwise use `alt` attribute, even if its value is the empty string.
+              If the <a data-cite="accname-1.2/#dfn-accessible-name">accessible name</a> is still empty: use `alt` attribute, even if its value is the empty string.
               <div class="note">
                 An `img` with an `alt` attribute whose value is the empty string is mapped to the
                 <a class="core-mapping" href="#role-map-presentation">`presentation`</a> role. It has no accessible name.
               </div>
             </li>
-            <li>Otherwise, if there is no `alt` attribute use the `title` attribute.</li>
+            <li>If the <a data-cite="accname-1.2/#dfn-accessible-name">accessible name</a> is still empty and there is no `alt` attribute, use the `title` attribute.</li>
+            <li>If the <a data-cite="accname-1.2/#dfn-accessible-name">accessible name</a> is still empty and there are no `alt` or `title` attributes, and the `img` is a descendent of a `figure` element with a child `figcaption` but no other <a data-cite="html/dom.html#flow-content-2">flow content</a> or <a data-cite="html/dom.html#inter-element-whitespace">whitespace</a> descendents, then use the <a data-cite="accname-1.2/#mapping_additional_nd_te">text equivalent computation</a> of the `figcaption` element's subtree.  
             <li>Otherwise there is no <a data-cite="accname-1.2/#dfn-accessible-name">accessible name</a>.</li>
           </ol>
         </section>