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.

src/renderer/atlas/CharAtlasCache.ts

@@ -0,0 +1,76 @@
+/**
+ * Copyright (c) 2017 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+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';
+
+interface ICharAtlasCacheEntry {
+  atlas: BaseCharAtlas;
+  config: ICharAtlasConfig;
+  // N.B. This implementation potentially holds onto copies of the terminal forever, so
+  // this may cause memory leaks.
+  ownedBy: ITerminal[];
+}
+
+const charAtlasCache: ICharAtlasCacheEntry[] = [];
+
+/**
+ * Acquires a char atlas, either generating a new one or returning an existing
+ * one that is in use by another terminal.
+ * @param terminal The terminal.
+ * @param colors The colors to use.
+ */
+export function acquireCharAtlas(
+  terminal: ITerminal,
+  colors: IColorSet,
+  scaledCharWidth: number,
+  scaledCharHeight: number,
+): BaseCharAtlas {
+  const newConfig = generateConfig(scaledCharWidth, scaledCharHeight, terminal, colors);
+
+  // Check to see if the terminal already owns this config
+  for (let i = 0; i < charAtlasCache.length; i++) {
+    const entry = charAtlasCache[i];
+    const ownedByIndex = entry.ownedBy.indexOf(terminal);
+    if (ownedByIndex >= 0) {
+      if (configEquals(entry.config, newConfig)) {
+        return entry.atlas;
+      } else {
+        // The configs differ, release the terminal from the entry
+        if (entry.ownedBy.length === 1) {
+          charAtlasCache.splice(i, 1);
+        } else {
+          entry.ownedBy.splice(ownedByIndex, 1);
+        }
+        break;
+      }
+    }
+  }
+
+  // Try match a char atlas from the cache
+  for (let i = 0; i < charAtlasCache.length; i++) {
+    const entry = charAtlasCache[i];
+    if (configEquals(entry.config, newConfig)) {
+      // Add the terminal to the cache entry and return
+      entry.ownedBy.push(terminal);
+      return entry.atlas;
+    }
+  }
+
+  const newEntry: ICharAtlasCacheEntry = {
+    atlas: new StaticCharAtlas(
+      document,
+      newConfig,
+    ),
+    config: newConfig,
+    ownedBy: [terminal],
+  };
+  charAtlasCache.push(newEntry);
+  return newEntry.atlas;
+}