Fix underbrace (#1304)

* Fix \underbrace

Fix the vertical alignment so that it correctly takes into account the descender of the lower element.

Fixes issue 1303.

* Update screenshots

src/buildHTML.js

@@ -493,7 +493,8 @@ export const groupTypes = {
             } else {
                 vlist = buildCommon.makeVList({
                     positionType: "bottom",
-                    positionData: vSpan.depth + 0.2 + supSubGroup.height,
+                    positionData: vSpan.depth + 0.2 + supSubGroup.height +
+                        supSubGroup.depth,
                     children: [
                         {type: "elem", elem: supSubGroup},
                         {type: "kern", size: 0.2},