Add BaseCharAtlas and implement StaticCharAtlas

The API defined by BaseCharAtlas will let us support multiple char atlas
implementations at once.

I tested this by running the demo with Chrome's profiler, which shows
that calls are going to StaticCharAtlas/drawImage instead of
_drawUncachedChar. Applications using 256 colors also still work fine,
via _drawUncachedChar.

tsconfig.json

@@ -2,6 +2,7 @@
   "compilerOptions": {
     "module": "commonjs",
     "target": "es5",
+    "lib": ["DOM", "ES5", "ScriptHost", "ES2015.Promise"],
     "rootDir": "src",
     "outDir": "lib",
     "sourceMap": true,