Add API to clear canvas renderer texture atlas

Fixes #3455

src/browser/renderer/atlas/BaseCharAtlas.ts

@@ -28,6 +28,8 @@ export abstract class BaseCharAtlas implements IDisposable {
    */
   private _doWarmUp(): void { }
 
+  public clear(): void { }
+
   /**
    * Called when we start drawing a new frame.
    *