Add overflow: auto to line widget wrapper to prevent margin leaking

(If the widget had a margin, that margin would end up outside the wrapper, throwing off
line height measurement.)

Might be related to issue #925

lib/codemirror.css

@@ -196,6 +196,7 @@ div.CodeMirror span.CodeMirror-nonmatchingbracket {color: #f22;}
 .CodeMirror-linewidget {
   position: relative;
   z-index: 2;
+  overflow: auto;
 }
 
 .CodeMirror-wrap .CodeMirror-scroll {