Split AlphaThresholdEffect out into its own file

tests/FlattenableFactoryToName.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAlphaThresholdFilter.h"
 #include "SkImage.h"
+#include "SkRegion.h"
 #include "Test.h"
 
 static void test_flattenable(skiatest::Reporter* r,
                              const SkFlattenable* f,
                              const char* desc) {
     if (f) {
         SkFlattenable::Factory factory = f->getFactory();
         REPORTER_ASSERT(r, factory);
         if (factory) {
             if (!SkFlattenable::FactoryToName(factory)) {
@@ skipping 12 matching lines @@
     sk_sp<SkImageFilter> filter(SkAlphaThresholdFilter::Make(region, 0.2f, 0.7f, nullptr));
     test_flattenable(r, filter.get(), "SkAlphaThresholdFilter()");
 
     SkBitmap bm;
     bm.allocN32Pixels(8, 8);
     bm.eraseColor(SK_ColorCYAN);
     sk_sp<SkImage> image(SkImage::MakeFromBitmap(bm));
     auto shader = image->makeShader(SkShader::kClamp_TileMode, SkShader::kClamp_TileMode);
     test_flattenable(r, shader.get(), "SkImage::newShader()");
 }