message: fix canvas not cleared
  happens e.g. with antialiasing disabled plus transparent background on
diff: @@ -290,13 +290,18 @@ export class DrawPass {
  renderer.setViewport(x, y, width, height);
  renderer.update(camera);
  + if (transparentBackground) {
  + this.drawTarget.bind();
  + renderer.clear(false);
  + }
  +
  if (this.wboitEnabled) {
  this._renderWboit(renderer, camera, scene, transparentBackground, postprocessingProps);
  } else {
  this._renderBlended(renderer, camera, scene, !volumeRendering && !postprocessingEnabled && !antialiasingEnabled && toDrawingBuffer, transparentBackground, postprocessingProps);
  }
  - if (PostprocessingPass.isEnabled(postprocessingProps)) {
  + if (postprocessingEnabled) {
  this.postprocessing.target.bind();
  } else if (!toDrawingBuffer || volumeRendering || this.wboitEnabled) {
  this.colorTarget.bind();
  @@ -323,7 +328,7 @@ export class DrawPass {
  this.drawTarget.bind();
  this.webgl.state.disable(this.webgl.gl.DEPTH_TEST);
  - if (PostprocessingPass.isEnabled(postprocessingProps)) {
  + if (postprocessingEnabled) {
  this.copyFboPostprocessing.render();
  } else if (volumeRendering || this.wboitEnabled) {
  this.copyFboTarget.render();
  