Add a per-frame limit on dynamic atlas writes

This limits the amount of damage a pathological program can cause in
thrashing the LRU cache.

I ran the script described here:
#1327 (comment)

Without this patch, the entire output would get drawn in one frame that
took about 140 ms to draw. With this patch, it takes about 60 ms. It's
still nowhere as good as the static or none atlas implementations, but
it's not terrible, like it was.

This should have minimal impact on real-world applications: it'll just
take a little longer for the atlas to warm up.

The implementation is a little hacky, since it involves throwing some
code into TextRenderLayer that violates the separation of concerns, but
we can clean this up later.

src/renderer/BaseRenderLayer.ts

@@ -20,7 +20,7 @@ export abstract class BaseRenderLayer implements IRenderLayer {
   private _scaledCharLeft: number = 0;
   private _scaledCharTop: number = 0;
 
-  private _charAtlas: BaseCharAtlas;
+  protected _charAtlas: BaseCharAtlas;
 
   constructor(
     private _container: HTMLElement,

src/renderer/TextRenderLayer.ts

@@ -54,6 +54,8 @@ export class TextRenderLayer extends BaseRenderLayer {
       return;
     }
 
+    this._charAtlas.beginFrame();
+
     for (let y = startRow; y <= endRow; y++) {
       const row = y + terminal.buffer.ydisp;
       const line = terminal.buffer.lines.get(row);

src/renderer/atlas/BaseCharAtlas.ts

@@ -25,6 +25,15 @@ export default abstract class BaseCharAtlas {
    */
   protected _doWarmUp(): void { }
 
+  /**
+   * Called when we start drawing a new frame.
+   *
+   * TODO: We rely on this getting called by TextRenderLayer. This should really be called by
+   * Renderer instead, but we need to make Renderer the source-of-truth for the char atlas, instead
+   * of BaseRenderLayer.
+   */
+  public beginFrame(): void { }
+
   /**
    * May be called before warmUp finishes, however it is okay for the implementation to
    * do nothing and return false in that case.

src/renderer/atlas/DynamicCharAtlas.ts

@@ -14,6 +14,14 @@ import LRUMap from './LRUMap';
 const TEXTURE_WIDTH = 1024;
 const TEXTURE_HEIGHT = 1024;
 
+// Drawing to the cache is expensive: If we have to draw more than this number of glyphs to the
+// cache in a single frame, give up on trying to cache anything else, and try to finish the current
+// frame ASAP.
+//
+// This helps to limit the amount of damage a program can do when it would otherwise thrash the
+// cache.
+const DRAW_TO_CACHE_LIMIT = 100;
+
 interface IGlyphCacheValue {
   index: number;
   isEmpty: boolean;
@@ -39,6 +47,8 @@ export default class DynamicCharAtlas extends BaseCharAtlas {
   private _width: number;
   private _height: number;
 
+  private _drawToCacheCount: number = 0;
+
   constructor(document: Document, private _config: ICharAtlasConfig) {
     super();
     this._cacheCanvas = document.createElement('canvas');
@@ -64,6 +74,10 @@ export default class DynamicCharAtlas extends BaseCharAtlas {
     // document.body.appendChild(this._cacheCanvas);
   }
 
+  public beginFrame(): void {
+    this._drawToCacheCount = 0;
+  }
+
   public draw(
     ctx: CanvasRenderingContext2D,
     glyph: IGlyphIdentifier,
@@ -75,7 +89,7 @@ export default class DynamicCharAtlas extends BaseCharAtlas {
     if (cacheValue != null) {
       this._drawFromCache(ctx, cacheValue, x, y);
       return true;
-    } else if (this._canCache(glyph)) {
+    } else if (this._canCache(glyph) && this._drawToCacheCount < DRAW_TO_CACHE_LIMIT) {
       let index;
       if (this._cacheMap.size < this._cacheMap.capacity) {
         index = this._cacheMap.size;
@@ -137,6 +151,7 @@ export default class DynamicCharAtlas extends BaseCharAtlas {
   // TODO: We do this (or something similar) in multiple places. We should split this off
   // into a shared function.
   private _drawToCache(glyph: IGlyphIdentifier, index: number): IGlyphCacheValue {
+    this._drawToCacheCount++;
 
     // draw the background
     let backgroundColor;