Remove substring in invoking _layoutEmoji.

Runtime/ui/txt/layout.cs

 
             char startingChar = buff.charAt(start);
             if (char.IsHighSurrogate(startingChar) || EmojiUtils.isSingleCharEmoji(startingChar)) {
-                advance = _layoutEmoji(buff.text.Substring(buff.offset + start, count), style, font, count,
+                advance = _layoutEmoji(buff.text, buff.offset + start, style, font, count,
                     advances, positions, advanceOffset, advance);
             }
             else {
             return x;
         }
 
-        static float _layoutEmoji(string text, TextStyle style, Font font, int count, float[] advances,
+        static float _layoutEmoji(string text, int start, TextStyle style, Font font, int count, float[] advances,
-                char c = text[i];
+                char c = text[start + i];
                 if (EmojiUtils.isSingleCharNonEmptyEmoji(c) || char.IsHighSurrogate(c)) {
                     float letterSpace = style.letterSpacing;
                     float letterSpaceHalfLeft = letterSpace * 0.5f;