Clean up buffer clean up/fill logic

The alt buffer is now cleared immediated after activating the normal buffer and
is filled when switching to it. The tests were failing because the alt buffer
wasn't being cleared properly with the previous solution.

src/BufferSet.ts

@@ -22,6 +22,7 @@ export class BufferSet extends EventEmitter implements IBufferSet {
   constructor(private _terminal: ITerminal) {
     super();
     this._normal = new Buffer(this._terminal);
+    this._normal.fillViewportRows();
     this._alt = new Buffer(this._terminal);
     this._activeBuffer = this._normal;
   }
@@ -54,6 +55,11 @@ export class BufferSet extends EventEmitter implements IBufferSet {
    * Sets the normal Buffer of the BufferSet as its currently active Buffer
    */
   public activateNormalBuffer(): void {
+    // The alt buffer should always be cleared when we switch to the normal
+    // buffer. This frees up memory since the alt buffer should always be new
+    // when activated.
+    this._alt.clear();
+
     this._activeBuffer = this._normal;
     this.emit('activate', this._normal);
   }
@@ -62,6 +68,10 @@ export class BufferSet extends EventEmitter implements IBufferSet {
    * Sets the alt Buffer of the BufferSet as its currently active Buffer
    */
   public activateAltBuffer(): void {
+    // Since the alt buffer is always cleared when the normal buffer is
+    // activated, we want to fill it when switching to it.
+    this._alt.fillViewportRows();
+
     this._activeBuffer = this._alt;
     this.emit('activate', this._alt);
   }