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/SelectionModel.test.ts

@@ -5,6 +5,7 @@ import { assert } from 'chai';
 import { ITerminal } from './Interfaces';
 import { SelectionModel } from './SelectionModel';
 import {BufferSet} from './BufferSet';
+import { MockTerminal } from './utils/TestUtils';
 
 class TestSelectionModel extends SelectionModel {
   constructor(
@@ -22,7 +23,9 @@ describe('SelectionManager', () => {
   let model: TestSelectionModel;
 
   beforeEach(() => {
-    terminal = <any>{ cols: 80, rows: 2, ybase: 0 };
+    terminal = new MockTerminal();
+    terminal.cols = 80;
+    terminal.rows = 2;
     terminal.scrollback = 10;
     terminal.buffers = new BufferSet(terminal);
     terminal.buffer = terminal.buffers.active;