Fixes for unicode in Web terminal

This is very partial, and my experience shows that the terminal
renders these poorly at best; in particular spacing in the DOM
model seems to be unpredictable with emoji and some of the esoteric
combining characters.  Still this represents a significant improvement
by actually including the combining characters.  Plus it is faster.

webfiles/tcell.js

@@ -61,12 +61,7 @@ function clearScreen(fg, bg) {
   }
 }
 
-function drawCell(x, y, mainc, combc, fg, bg, attrs, us, uc) {
-  var combString = String.fromCharCode(mainc);
-  combc.forEach((char) => {
-    combString += String.fromCharCode(char);
-  });
-
+function drawCell(x, y, s, fg, bg, attrs, us, uc) {
   var span = document.createElement("span");
   var use = false;
 
@@ -123,11 +118,11 @@ function drawCell(x, y, mainc, combc, fg, bg, attrs, us, uc) {
   if ((attrs & (1 << 1)) != 0) {
     var blink = document.createElement("span");
     blink.classList.add("blink");
-    var textnode = document.createTextNode(combString);
+    var textnode = document.createTextNode(s);
     blink.appendChild(textnode);
     span.appendChild(blink);
   } else {
-    var textnode = document.createTextNode(combString);
+    var textnode = document.createTextNode(s);
     span.appendChild(textnode);
   }
 

wscreen.go

@@ -143,13 +143,18 @@ func (t *wScreen) drawCell(x, y int) int {
 		uc = 0x000000
 	}
 
-	var combcarr []interface{} = make([]interface{}, len(combc))
-	for i, c := range combc {
-		combcarr[i] = c
+	s := ""
+	if len(combc) > 0 {
+		b := make([]rune, 0, 1 + len(combc))
+		b = append(b, mainc)
+		b = append(b, combc...)
+		s = string(b)
+	} else {
+		s = string(mainc)
 	}
 
 	t.cells.SetDirty(x, y, false)
-	js.Global().Call("drawCell", x, y, mainc, combcarr, fg, bg, int(style.attrs), int(us), int(uc))
+	js.Global().Call("drawCell", x, y, s, fg, bg, int(style.attrs), int(us), int(uc))
 
 	return width
 }