Implement onRender in xterm-headless

This effectively enabled implementing an efficient renderer on top
of xterm-headless. See #5392

src/common/CoreTerminal.ts

@@ -65,6 +65,8 @@ export abstract class CoreTerminal extends Disposable implements ICoreTerminal {
   public readonly onData = this._onData.event;
   protected _onLineFeed = this._register(new Emitter<void>());
   public readonly onLineFeed = this._onLineFeed.event;
+  protected readonly _onRender = this._register(new Emitter<{ start: number, end: number }>());
+  public readonly onRender = this._onRender.event;
   private readonly _onResize = this._register(new Emitter<{ cols: number, rows: number }>());
   public readonly onResize = this._onResize.event;
   protected readonly _onWriteParsed = this._register(new Emitter<void>());
@@ -124,6 +126,7 @@ export abstract class CoreTerminal extends Disposable implements ICoreTerminal {
     // Register input handler and handle/forward events
     this._inputHandler = this._register(new InputHandler(this._bufferService, this._charsetService, this.coreService, this._logService, this.optionsService, this._oscLinkService, this.coreMouseService, this.unicodeService));
     this._register(Event.forward(this._inputHandler.onLineFeed, this._onLineFeed));
+    this._register(Event.forward(Event.map(this._inputHandler.onRequestRefreshRows, e => ({ start: e?.start ?? 0, end: e?.end ?? this.rows - 1 })), this._onRender));
     this._register(this._inputHandler);
 
     // Setup listeners