Optimize layout: request character only at begining.

Runtime/ui/txt/layout.cs

             }
             else {
                 Font font = FontManager.instance.getOrCreate(style.fontFamily, style.fontWeight, style.fontStyle).font;
-                font.RequestCharactersInTextureSafe(text, style.UnityFontSize, style.UnityFontStyle);
+                // font.RequestCharactersInTextureSafe(text, style.UnityFontSize, style.UnityFontStyle);
                 for (int i = 0; i < text.Length; i++) {
                     char ch = text[i];
                     if (font.getGlyphInfo(ch, out var glyphInfo, style.UnityFontSize, style.UnityFontStyle)) {
             else {
                 Font font = FontManager.instance.getOrCreate(style.fontFamily, style.fontWeight, style.fontStyle).font;
                 // TODO: it is kind of a waste to require the entire string for this style, but SubString causes alloc
-                font.RequestCharactersInTextureSafe(text, style.UnityFontSize, style.UnityFontStyle);
+                // font.RequestCharactersInTextureSafe(text, style.UnityFontSize, style.UnityFontStyle);
                 for (int i = 0; i < count; i++) {
                     char ch = text[start + i];
                     if (ch == '\t') {

Runtime/ui/txt/paragraph.cs

             return nextLineStart - lineStart;
         }
 
-        internal void setText(string text, StyledRuns runs) {
+        internal void setText(string text, StyledRuns runs, bool skipRequestCharacters = false) {
+            if (!skipRequestCharacters) {
+                for (int i = 0; i < runs.size; i++) {
+                    var style = runs.getStyle(i);
+                    var run = runs.getRun(i);
+                    var font = FontManager.instance.getOrCreate(style.fontFamily, style.fontWeight, style.fontStyle).font;
+                    font.RequestCharactersInTextureSafe(text.Substring(run.start, run.end - run.start),
+                        Mathf.CeilToInt(style.fontSize), style.UnityFontStyle);
+                }
+            }
         }
 
         public void setParagraphStyle(ParagraphStyle style) {