MDL-38355 qtype_multianswer: input boxes too big in Russian.

... and any other language where the characters take multiple
bytes in UTF-8.

This fix was worked out by Dušan Ristić. I am just committing it.

question/type/multianswer/renderer.php

@@ -189,9 +189,9 @@ public function subquestion(question_attempt $qa, question_display_options $opti
         }
 
         // Work out a good input field size.
-        $size = max(1, strlen(trim($response)) + 1);
+        $size = max(1, textlib::strlen(trim($response)) + 1);
         foreach ($subq->answers as $ans) {
-            $size = max($size, strlen(trim($ans->answer)));
+            $size = max($size, textlib::strlen(trim($ans->answer)));
         }
         $size = min(60, round($size + rand(0, $size*0.15)));
         // The rand bit is to make guessing harder.