Add a dummy NoneCharAtlas implementation

This adds the logic needed to switch between multiple char atlas
implementations, and an example second implementation to test it out
with.

This also offers an escape hatch if something goes wrong with the char
atlas implementation(s).

src/renderer/atlas/CharAtlasCache.ts

@@ -4,11 +4,17 @@
  */
 
 import { ITerminal } from '../../Types';
-import BaseCharAtlas from './BaseCharAtlas';
-import StaticCharAtlas from './StaticCharAtlas';
 import { IColorSet } from '../Types';
 import { ICharAtlasConfig } from '../../shared/atlas/Types';
 import { generateConfig, configEquals } from './CharAtlasUtils';
+import BaseCharAtlas from './BaseCharAtlas';
+import NoneCharAtlas from './NoneCharAtlas';
+import StaticCharAtlas from './StaticCharAtlas';
+
+const charAtlasImplementations = {
+  'none': NoneCharAtlas,
+  'static': StaticCharAtlas,
+};
 
 interface ICharAtlasCacheEntry {
   atlas: BaseCharAtlas;
@@ -64,7 +70,7 @@ export function acquireCharAtlas(
   }
 
   const newEntry: ICharAtlasCacheEntry = {
-    atlas: new StaticCharAtlas(
+    atlas: new charAtlasImplementations[terminal.options.experimentalCharAtlas](
       document,
       newConfig,
     ),

src/renderer/atlas/CharAtlasUtils.ts

@@ -17,6 +17,7 @@ export function generateConfig(scaledCharWidth: number, scaledCharHeight: number
     ansi: colors.ansi.slice(0, 16)
   };
   return {
+    type: terminal.options.experimentalCharAtlas,
     devicePixelRatio: window.devicePixelRatio,
     scaledCharWidth,
     scaledCharHeight,
@@ -35,7 +36,8 @@ export function configEquals(a: ICharAtlasConfig, b: ICharAtlasConfig): boolean
       return false;
     }
   }
-  return a.devicePixelRatio === b.devicePixelRatio &&
+  return a.type === b.type &&
+      a.devicePixelRatio === b.devicePixelRatio &&
       a.fontFamily === b.fontFamily &&
       a.fontSize === b.fontSize &&
       a.fontWeight === b.fontWeight &&

src/renderer/atlas/NoneCharAtlas.ts

@@ -0,0 +1,25 @@
+/**
+ * Copyright (c) 2017 The xterm.js authors. All rights reserved.
+ * @license MIT
+ *
+ * A dummy CharAtlas implementation that always fails to draw characters.
+ */
+
+import { IGlyphIdentifier } from './Types';
+import { ICharAtlasConfig } from '../../shared/atlas/Types';
+import BaseCharAtlas from './BaseCharAtlas';
+
+export default class NoneCharAtlas extends BaseCharAtlas {
+  constructor(document: Document, config: ICharAtlasConfig) {
+    super();
+  }
+
+  public draw(
+    ctx: CanvasRenderingContext2D,
+    glyph: IGlyphIdentifier,
+    x: number,
+    y: number
+  ): boolean {
+    return false;
+  }
+}