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,
     ),