use special-image for imagefilters and save/restore layer

src/pdf/SkPDFDevice.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkPDFDevice.h"
 #include "SkAnnotationKeys.h"
 #include "SkBitmapDevice.h"
@@ skipping 2244 matching lines @@
 sk_sp<SkSpecialImage> SkPDFDevice::makeSpecial(const SkBitmap& bitmap) {
     return SkSpecialImage::MakeFromRaster(bitmap.bounds(), bitmap);
 }
 
 sk_sp<SkSpecialImage> SkPDFDevice::makeSpecial(const SkImage* image) {
     return SkSpecialImage::MakeFromImage(SkIRect::MakeWH(image->width(), image->height()),
                                          image->makeNonTextureImage());
 }
 
 sk_sp<SkSpecialImage> SkPDFDevice::snapSpecial() {
-    SkASSERT(false);
     return nullptr;
 }