KaTeX: Fix bug with embedded SVGs

See https://github.com/Jermolene/TiddlyWiki5/issues/2500#issuecomment-489976530

plugins/tiddlywiki/katex/styles.tid

@@ -13,6 +13,12 @@ tags: [[$:/tags/Stylesheet]]
     text-rendering: auto;
 }
 
+/* Avoid TW5's max-width: 100% */
+
+.katex svg {
+  max-width: initial;
+}
+
 /* Override font URLs */
 
 @font-face {