Fix emoji size.

Author: Yuncong Zhang

Runtime/ui/txt/emoji.cs

@@ -235,7 +235,7 @@ public static Image image {
         public const int colCount = 37;
 
         public static Rect getMinMaxRect(float fontSize, float ascent, float descent) {
-            return Rect.fromLTWH(fontSize * 0.05f, descent - fontSize, fontSize * 0.9f, fontSize * 0.9f);
+            return Rect.fromLTWH(0, descent/2-fontSize, fontSize, fontSize);
         }
 
         public static Rect getUVRect(int code) {

Runtime/ui/txt/layout.cs

@@ -226,9 +226,9 @@ static float _layoutEmoji(string text, int start, int count, TextStyle style, Fo
 
 
                     var minX = x;
-                    var maxX = metrics.descent - metrics.ascent + x;
-                    var minY = metrics.ascent;
-                    var maxY = metrics.descent;
+                    var maxX = style.fontSize + x;
+                    var minY = metrics.descent/2 - style.fontSize;
+                    var maxY = metrics.descent/2;
                     _updateBounds(minX, maxX, minY, maxY, ref bounds);
 
                     positions[i] = x;