message: FilterManager:
  don't clear the entire target buffer when popping the last filter
  fixes issues with applying multiple filters to a single object
diff: @@ -184,7 +184,7 @@ export default class FilterManager extends WebGLManager
  flop = t;
  }
  - filters[i].apply(this, flip, lastState.renderTarget, true);
  + filters[i].apply(this, flip, lastState.renderTarget, false);
  this.freePotRenderTarget(flip);
  this.freePotRenderTarget(flop);
  