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:
https://github.com/xtermjs/xterm.js/pull/1327#issuecomment-374003960

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/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.