Split AlphaThresholdEffect out into its own file

include/effects/SkAlphaThresholdFilter.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkAlphaThresholdFilter_DEFINED
 #define SkAlphaThresholdFilter_DEFINED
 
 #include "SkImageFilter.h"
-#include "SkRegion.h"
+
+class SkRegion;
 
 class SK_API SkAlphaThresholdFilter {
 public:
     /**
      * Creates an image filter that samples a region. If the sample is inside the
      * region the alpha of the image is boosted up to a threshold value. If it is
      * outside the region then the alpha is decreased to the threshold value.
      * The 0,0 point of the region corresponds to the upper left corner of the
      * source image.
      */
     static sk_sp<SkImageFilter> Make(const SkRegion& region, SkScalar innerMin,
                                      SkScalar outerMax, sk_sp<SkImageFilter> input,
                                      const SkImageFilter::CropRect* cropRect = nullptr);
 
 
 #ifdef SK_SUPPORT_LEGACY_IMAGEFILTER_PTR
     static SkImageFilter* Create(const SkRegion& region, SkScalar innerMin,
                                  SkScalar outerMax, SkImageFilter* input = nullptr) {
         return Make(region, innerMin, outerMax, sk_ref_sp<SkImageFilter>(input)).release();
     }
 #endif
 
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP();
 };
 
 #endif