Fix \vec (#1018)

* Fix \vec

In accent \vec, replace the combining font glyph with an SVG.

* Fix lint error

* update screenshots containing vectors

* update HorizontalBraces

static/katex.less

@@ -558,19 +558,16 @@
             width: 0;
         }
 
-        .accent-body.accent-vec > span {
-            position: relative;
-            // This value is half of the value that the MathJax's makeFF shifts
-            // it left. We center it by shifting it half way right again.
-            left: 0.326em;
-        }
-
         .accent-body.accent-hungarian > span {
             position: relative;
             left: 0.250em; // width of space
         }
     }
 
+    .overlay {
+        display: block;
+    }
+
     .mtable {
         .vertical-separator {
             display: inline-block;