Improve CSI J functionality

CSI J is the escape sequence for clearing some part of the terminal
screen. The `clear` command uses this sequence to clear the terminal.
Improve how the sequence clears the screen by preserving the previous
value of the cursor in CSI H, the sequence for setting cursor position.

src/term.js

@@ -3615,6 +3615,7 @@ Terminal.prototype.cursorPos = function(params) {
     col = this.cols - 1;
   }
 
+  this.prevY = this.y;
   this.x = col;
   this.y = row;
 };
@@ -3648,7 +3649,7 @@ Terminal.prototype.eraseInDisplay = function(params) {
       }
       break;
     case 2:
-      j = this.rows;
+      j = this.prevY || this.rows;
       while (j--)
       {
         this.lines.push(this.blankLine());