Add a basic dynamic character atlas implementation

This adds a very minimal implementation of DynamicCharAtlas using a LRU
cache.

I've got some optimizations that I'll add on top of this, but this
proves the concept.

src/renderer/atlas/CharAtlasCache.ts

@@ -8,12 +8,14 @@ import { IColorSet } from '../Types';
 import { ICharAtlasConfig } from '../../shared/atlas/Types';
 import { generateConfig, configEquals } from './CharAtlasUtils';
 import BaseCharAtlas from './BaseCharAtlas';
+import DynamicCharAtlas from './DynamicCharAtlas';
 import NoneCharAtlas from './NoneCharAtlas';
 import StaticCharAtlas from './StaticCharAtlas';
 
 const charAtlasImplementations = {
   'none': NoneCharAtlas,
   'static': StaticCharAtlas,
+  'dynamic': DynamicCharAtlas,
 };
 
 interface ICharAtlasCacheEntry {